Brain Uptake and Utilization of Fatty Acids,Lipids and Lipoproteins: Application to Neurological Disorders

Transport, synthesis, and utilization of brain fatty acids and other lipids have been topics of investigation for more than a century, yet many fundamental aspects are unresolved and, indeed, subject to controversy. Understanding the mechanisms by which lipids cross the blood brain barrier and how they are utilized by neurons and glia is critical to understanding normal brain development and function, for the diagnosis and therapy of human diseases, and for the planning and delivery of optimal human nutrition throughout the world. Two particularly important fatty acids, both of which are abundant in neuronal membranes are: (a) the ω3 polyunsaturated fatty acid docosahexaenoic acid, deficiencies of which can impede brain development and compromise optimal brain function, and (b) the ω6 polyunsaturated fatty acid arachidonic acid, which yields essential, but potentially toxic, metabolic products. There is an exciting emerging evidence that modulating dietary intake of these fatty acids could have a beneficial effect on human neurological health. A workshop was held in October, 2004, in which investigators from diverse disciplines interacted to present new findings and to discuss issues relevant to lipid uptake, utilization, and metabolism in the brain. The objectives of this workshop were: (1) to assess the state-of-the-art of research in brain fatty acid/lipid uptake and utilization; (2) to discuss progress in understanding molecular mechanisms and the treatment of neurological diseases related to lipids and lipoproteins; (3) to identify areas in which current knowledge is insufficient; (4) to provide recommendations for future research; and (5) to stimulate the interest and involvement of additional neuroscientists, particularly young scientists, in these areas. The meeting was divided into four sessions: (1) mechanisms of lipid uptake and transport in the brain, (2) lipoproteins and polyunsaturated fatty acids, (3) eicosanoids in brain function, and (4) fatty acids and lipids in brain disorders. In this article, we will provide an overview of the topics discussed in these sessions.     

Brain uptake and utilization of fatty acids

The brain is rich in diverse fatty acids saturated, monounsaturated and polyunsaturated fatty acids with chain lengths ranging from less than 16 to more than 24 carbons that make up the complex lipids present in this organ. While some fatty acids are derived from endogenous synthesis, others must come from exogenous sources. The mechanism(s) by which fatty acids enter cells has been the subject of much debate. While some investigators argue for a protein-mediated process, others suggest that simple diffusion is sufficient. In the brain, uptake is further complicated by the presence of the blood-brain barrier. Brain fatty acid homeostasis is disturbed in many human disorders, as typified by the peroxisomal biogenesis diseases. A workshop designed to bring together researchers from varied backgrounds to discuss these issues in an open forum was held in March, 2000. In addition to assessing the current state of knowledge, areas requiring additional investigation were identified and recommendations for future research were made. A brief overview of the invited talks is presented here.     

Brain uptake and utilization of fatty acids: applications to peroxisomal biogenesis diseases

The brain is rich in diverse fatty acids saturated, monounsaturated and polyunsaturated fatty acids with chain lengths ranging from less than 16 to more than 24 carbons that make up the complex lipids present in this organ. While some fatty acids are derived from endogenous synthesis, others must come from exogenous sources. The mechanism(s) by which fatty acids enter cells has been the subject of much debate. While some investigators argue for a protein-mediated process, others suggest that simple diffusion is sufficient. In the brain, uptake is further complicated by the presence of the blood-brain barrier. Brain fatty acid homeostasis is disturbed in many human disorders, as typified by the peroxisomal biogenesis diseases. A workshop designed to bring together researchers from varied backgrounds to discuss these issues in an open forum was held in March, 2000. In addition to assessing the current state of knowledge, areas requiring additional investigation were identified and recommendations for future research were made. A brief overview of the invited talks is presented here.     

Plasma and brain fatty acid profiles in mild cognitive impairment and Alzheimer's disease

Alzheimer's disease (AD) is generally associated with lower omega-3 fatty acid intake from fish but despite numerous studies, it is still unclear whether there are differences in omega-3 fatty acids in plasma or brain. In matched plasma and brain samples provided by the Memory and Aging Project, fatty acid profiles were quantified in several plasma lipid classes and in three brain cortical regions. Fatty acid data were expressed as % composition and as concentrations (mg/dL for plasma or mg/g for brain). Differences in plasma fatty acid profiles between AD, mild cognitive impairment (MCI), and those with no cognitive impairment (NCI) were most apparent in the plasma free fatty acids (lower oleic acid isomers and omega-6 fatty acids in AD) and phospholipids (lower omega-3 fatty acids in AD). In brain, % DHA was lower only in phosphatidylserine of mid-frontal cortex and superior temporal cortex in AD compared to NCI (-14% and -12%, respectively; both p < 0.05). The only significant correlation between plasma and brain fatty acids was between % DHA in plasma total lipids and % DHA in phosphatidylethanolamine of the angular gyrus, but only in the NCI group (+0.77, p < 0.05). We conclude that AD is associated with altered plasma status of both DHA and other fatty acids unrelated to DHA, and that the lipid class-dependent nature of these differences reflects a combination of differences in intake and metabolism.     

In vivo fatty acid incorporation into brain phosholipids in relation to plasma availability, signal transduction and membrane remodeling

A method, model, and “operational equations” are described to quantify in vivo turnover rates and half-lives of fatty acids within brain phospholipids, as well as rates of incorporation of these fatty acids into brain from plasma. In awake rats, recycling of fatty acids within brain phospholipids, due to deesterification and reesterification, is very rapid, with half-lives in some cases of minutes to hours. Plasma fatty acids make only a small contribution (2–4%) to the net quantity of fatty acids that are reesterified. This explains why many weeks are necessary to recover normal brain n-3 polyunsaturated fatty acid concentrations following their prolonged dietary deprivation. Changes in recycling of specific fatty acids in response to centrally acting drugs can help to identify enzyme targets for drug action. For example, recycling of arachidonate is specifically reduced by 80% in rats treated chronically with lithium, a drug effective against bipolar disorder; the effect reflects downregulation of gene expression of an arachidonate-specific phospholipase A₂. When combined with neuroimaging (quantitative autoradiography in rodents or positron-emission tomography [PET] in macaques or humans), intravenously injected radiolabeled fatty acids can be used to localize and quantify brain PLA₂-mediated signal transduction, and to examine neuroplastic remodeling of brain lipid membranes.     

Intracranial microinfusion of pancreastatin elevates blood glucose, free fatty acids, and corticosterone in rats

Pancreastatin, a novel peptide recently isolated from porcine pancreas, significantly inhibits insulin and somatostatin release and augments glucagon release from the isolated perfused rat pancreas. This implies a role for endogenous pancreatic pancreastatin in the regulation of blood glucose and free fatty acids, the two major metabolic fuels. Since many peptides have similar biological effects when administered centrally and peripherally, the effects of centrally administered pancreastatin on blood glucose and free fatty acids were examined in 3 studies. Corticosterone was also measured in two of these studies. Intraventricular microinfusion of pancreastatin significantly elevated blood glucose, free fatty acid, and corticosterone concentrations in a dose-related manner. None of these effects was seen after subcutaneous injection of the same doses. Centrally administered pancreastatin appears to produce its effects on glucose and free fatty acids through actions in the brain, and either the brain, the median eminence, and/or pituitary for corticosterone.     

Long-term effects of high lipid and high energy diet on serum lipid,brain fatty acid composition,and memory and learning ability in mice

Objective

It is well known that high lipid and high energy diet is harmful to health. But the different effects of high lipid diet composed of either saturated fatty acids or unsaturated fatty acids have not been distinguished.

Method

Eighteen pregnant C57BL/6j (22–25 g) mice were randomly divided into three groups of six each and fed with chow or high lipid diet composed of either flaxseed oil (chow diet 84%, cholesterol 0.2%, flaxseed oil 15.8%) or lard fat (chow diet 84%, cholesterol 0.2%, lard fat 15.8%). After weaning, the offspring were fed the same diet as their mothers were fed during the experiment, and their spatial memory and learning ability were evaluated by Morris water maze when they were 8 weeks old. Next, the blood and tissues were sampled when they were 9 weeks old. Serum lipids were determined using kits, and brain fatty acids were measured using a gas chromatograph.

Results

Compared to chow diet (control), high flaxseed oil diet (HFO) increased high density lipoprotein cholesterol level (HDL-C) in the mothers but not in offspring; high lard fat diet (HLF) increased serum total cholesterol level (TC) and low density lipoprotein cholesterol level (LDL-C) both in mothers and offspring. Brain fatty acids profile was altered by HLF compared with chow diet. Polyunsaturated fatty acids and long-chain polyunsaturated fatty acids content were significantly lower in the HLF group than in the control group, but saturated fatty acids content were significantly higher in HLF group than those in control group. The changed fatty acids composition affected the spatial memory and learning ability of adult offspring.

Conclusions

A long-term high lard diet increased offspring serum TC and LDL-C levels and affected the brain's fatty acid composition, and memory and learning ability. The polyunsaturated fatty acid content of the brain may be correlated with serum cholesterol levels.     

Maternal intake of cashew nuts accelerates reflex maturation and facilitates memory in the offspring

Essential fatty acids, being indispensable during the stages of pregnancy, lactation and infancy influence the transmission of nerve impulses and brain function, and cashew nuts are a good source of these fatty acids. The objective of this study was to evaluate the effects of cashew nut consumption on reflex development, memory and profile of fatty acids of rat offspring treated during pregnancy and lactation. The animals were divided into three groups: Control (CONT), treated with 7% lipid derived from soybean oil; Normolipidic (NL) treated with 7% lipids derived from cashew nuts; and Hyperlipidic (HL) treated with 20% lipids derived from cashew nuts. Reflex ontogeny, Open-field habituation test and the Object Recognition Test (ORT) were assessed. The profile of fatty acids in the brain was carried out when the animals were zero, 21 and 60 days old. Accelerated reflex maturation was observed in animals treated with cashew nuts (p < 0.05). NL presented better memory in the Open-field habituation test; the NL and HL showed improvement of short-term memory in the ORT, but long term damage in HL (p < 0.05). The results of the lipid profile of the brain at the end of the experiment showed an increase in levels of saturated fatty acids and less Docosahexaenoic acid (DHA) in animals of the HL. The data showed that maternal consumption of cashew nuts can accelerate reflex maturation and facilitate memory in offspring when offered in adequate quantities.     

Do essential fatty acids play a role in brain and behavioral development?

The membrane phospholipids of the brain contain high levels of polyunsaturated fatty acids (PUFA), particularly arachidonic acid, 20:4n-6 and docosahexaenoic acid, 22:6n-3. These long-chain PUFA are synthesized from their respective essential fatty acid (EFA) precursors, linoleic acid, 18:2n-6 and linolenic acid, 18:3n-3. Although the necessity of n-6 fatty acids for optimum growth has been established, a similar requirement for those of the n-3 family is less clear. The rapid accumulation of the long-chain n-3 PUFA in the brain during prenatal and preweaning development suggests that the provision of n-3 fatty acids to the developing brain may be necessary for normal growth and functional development. The intent of this review is to assess the experimental work which addresses this question, most of which has been conducted on rodents. The emphasis will be on studies which measure behavioral outcomes, and particular attention will be paid to methodological issues which affect the interpretation of these data. An integration of the research findings will be presented and discussed in light of possible implications for therapeutic interventions.     

Essential polyunsaturated fatty acids and the barrier to the brain: the components of a model for transport

Several areas of research have contributed to the establishment of a paradigm that meets the requirements for the selective uptake of essential polyunsaturated fatty acids (EPUFA) into brain. First, discrete studies have demonstrated that cholesterol and the nonessential fatty acids, (palmitic, oleic, stearic) do not enter the brain parenchyma. These studies demonstrated that the 18 carbon-monocarboxylic fatty acids, linoleic acid with two cis-double bonds entered brain, whereas oleic acid, with one cis-double bond, did not enter brain. It was concluded the entry of essential fatty acids into brain is accomplished in a highly selective and discrete manner. Further, the typical blood-borne lipoproteins do not traverse the endothelial cells of the capillary network and enter into the brain, otherwise cholesterol, palmitic, oleic, and stearic acids from blood would be located within brain. Second, several investigators have shown that the endothelial cells of the capillary network contain lipoprotein receptors, yet one conclusion is that the brain does not utilize low-density lipoprotein (LDL)-cholesterol. Third, recently, the existence and function of a significant number of distinctive trans-membrane monocarboxylic acid transporters, (MCTs) and fatty acid transport proteins (FATPs) have been described. No transporters have been described to date with the specificity necessary to transfer only EPUFA into brain. A blueprint with the minimal elements for delivery and selectivity is proposed. Lipoproteins enter the endothelial cells because the lipoprotein receptors are positioned on their luminal membrane. Essential fatty acid transporter(s) are positioned on the abluminal membrane of these endothelial cells to allow for the entry of EPUFA into brain. Within the endothelial cell there is opportunity for lipid management and transformation such that EPUFAs are selectively culled for delivery to the essential fatty acid transporter(s), which facilitates their transfer into brain.     

Adrenoleukodystrophy and variants. Clinical, neurophysiological and biochemical studies in patients and family members

Clinical, neurophysiological and biochemical studies were performed in patients with various forms of adrenoleukodystrophy (ALD) and their family members. The patients showed an abnormality in saturated very long chain fatty acids and in the somatosensory and brain stem auditory or visual evoked potentials. Female presumptive carriers without abnormal neurological manifestations also showed abnormality in the somatosensory or brain stem auditory evoked potentials and in saturated very long chain fatty acids. One ALD patient and his mother, a female carrier, had the decreased beta-galactosidase activity. The increase in saturated very long chain fatty acids was found, not only in sphingomyelin, but also in phosphatidylcholine and phosphatidylserine. Our results suggest that a generalized abnormal metabolism of VLFA and an abnormality in the central nervous system exist in our patients and female carriers.     

Essential polyunsaturated fatty acids and the barrier to the brain

Several areas of research have contributed to the establishment of a paradigm that meets the requirements for the selective uptake of essential polyunsaturated fatty acids (EPUFA) into brain. First, discrete studies have demonstrated that cholesterol and the nonessential fatty acids, (palmitic, oleic, stearic) do not enter the brain parenchyma. These studies demonstrated that the 18 carbon-monocarboxylic fatty acids, linoleic acid with two cis-double bonds entered brain, whereas oleic acid, with one cis-double bond, did not enter brain. It was concluded the entry of essential fatty acids into brain is accomplished in a highly selective and discrete manner. Further, the typical blood-borne lipoproteins do not traverse the endothelial cells of the capillary network and enter into the brain, otherwise cholesterol, palmitic, oleic, and stearic acids from blood would be located within brain. Second, several investigators have shown that the endothelial cells of the capillary network contain lipoprotein receptors, yet one conclusion is that the brain does not utilize low-density lipoprotein (LDL)-cholesterol. Third, recently, the existence and function of a significant number of distinctive trans-membrane monocarboxylic acid transporters, (MCTs) and fatty acid transport proteins (FATPs) have been described. No transporters have been described to date with the specificity necessary to transfer only EPUFA into brain. A blueprint with the minimal elements for delivery and selectivity is proposed. Lipoproteins enter the endothelial cells because the lipoprotein receptors are positioned on their luminal membrane. Essential fatty acid transporter(s) are positioned on the abluminal membrane of these endothelial cells to allow for the entry of EPUFA into brain. Within the endothelial cell there is opportunity for lipid management and transformation such that EPUFAs are selectively culled for delivery to the essential fatty acid transporter(s), which facilitates their transfer into brain.     

Adrenoleukodystrophy brain cholesteryl esters and other neutral lipids

Further examination of the neutral lipid fractions derived from brain tissue of two patients afflicted with adrenoleukodystrophy has demonstrated the presence not only of free cholesterol and cholesteryl ester, but also of appreciable free fatty acid and triglyceride. Using a gas-liquid chromatographic system normally employed for the analysis of long-chain fatty acids of galactolipids and sphingomyelin, it was possible to establish the presence of long-chain (>C₂₀) fatty acids in the the cholesteryl ester, free fatty acid and triglyceride fractions. Long-chain fatty acids were most abundant in the cholesteryl esters. Fatty acids identified by gas-liquid chromatography and gas chromatography-mass spectroscopy included normal saturated and monounsaturated fatty acids as large as C₃₄. Several unknown fatty acyl compounds, of as yet undetermined structure, were also observed. All investigations thus far would indicate that the pathogenesis of adrenoleukodystrophy is closely related to the aberrant metabolism of these long-chain fatty acids.     

Dietary essential fatty acids and gender-specific behavioral responses in cranially irradiated rats

Specific memory deficits, reduced intellectual processing speed, and a variety of social and behavioral problems have been implicated as long-term effects of cranial radiation therapy (CRT). These deficits are thought to be related to changes in brain cytology and structure associated with microvascular aberrations. N-3 fatty acids may serve as protectants in pediatric patients who receive CRT for brain tumors. Timed-pregnant rat dams were fed one of four diets that were identical in all respects, except for their essential fatty acid content. The dams were placed on these diets at the beginning of the third trimester of gestation and their pups remained on them throughout the study. The rats’ behavioral response as judged by acoustic startle response (ASR) and neurocognitive response (performance in a radial maze, RM) were evaluated in relation to diet, gender, and CRT. The following hypotheses were tested: (1) female rats will show greater CRT-induced neurocognitive and behavioral deficits; (2) dietary n-3 fatty acids will diminish CRT-induced neurocognitive and behavioral deficits; (3) gender-specific differences would be dampened by n-3 fatty acids in the diet. All three hypotheses were partially supported. These findings are discussed in light of the potential neuroprotective effects of n-3 fatty acids.     

Synthesis of arachidonoyl coenzyme A and docosahexaenoyl coenzyme A in synaptic plasma membranes of cerebrum and microsomes of cerebrum, cerebellum, and brain stem of rat brain

Synthesis of arachidonoyl CoA and docosahexaenoyl CoA in homogenates and microsomes from cerebrum, cerebellum, and brain stem and in synaptic plasma membranes from cerebrum of control rats and rats undergoing bicuculline-induced status epilepticus were studied. Arachidonoyl CoA synthesis was 3-5 times higher than docosahexaenoyl CoA in homogenates and microsomes. The synaptic plasma membranes showed only 1.5- to 2.5-fold higher activity. The presence of Triton X-100 (0.1%) in the incubation medium did not alter the activity of arachidonoyl CoA synthesis but did increase the synthesis of docosahexaenoyl CoA in homogenates, microsomes, and especially in synaptic plasma membranes. The synthesis of these polyenoic fatty acyl CoAs were 4-6 times higher in microsomes than in homogenates. Synaptic plasma membranes exhibited about the same amount of activity as homogenates in the synthesis of docosahexaenoyl CoA, but only half the activity of the latter in arachidonoyl CoA synthesis. The synthesis of arachidonyl CoA and docosahexaenoyl CoA in cerebral homogenates and microsomes was higher than that of cerebellum and brain stem. The apparent Km values for labeled arachidonic acid (17 microM) and docosahexaenoic acid (12 microM) in synaptic plasma membranes were lower than the values for microsomes isolated from different brain regions. The Vmax values were also 4-10 times lower. Microsomes from different regions did not differ in their apparent Km values, but did show variations in apparent Vmax values. Cerebellar microsomes showed lower Vmax values than the other two regions. The presence of Triton X-100 caused a significant decrease in the apparent Km values with little change in the Vmax values. Bicuculline-induced seizures did not alter the kinetic properties of arachidonoyl CoA and docosahexaenoyl CoA synthesis, except there was a significant decrease in the apparent Km and Vmax values for cerebellar microsomal docosahexaenoyl CoA synthesis. In conclusion, there were marked differences in the activation of polyenoic fatty acids in different parts of the brain and in subcellular fractions. Although bicuculline-induced convulsions accumulate free polyenoic fatty acids in the brain, no changes were detected when the fatty activation was assayed with exogenous cofactors, except in cerebellum.     

Plasma free fatty acid and lipoproteins as sources of polyunsaturated fatty acid for the brain

Polyunsaturated fatty acids (PUFA), which comprise 25-30% of the fatty acids in the human brain, are necessary for normal brain development and function. PUFA cannot be synthesized de novo and must be supplied to the brain by the plasma. It is necessary to know the PUFA content and composition of the various plasma lipids and lipoproteins in order to understand how these fatty acids are taken up and metabolized by the brain. Human plasma free fatty acid (FFA) ordinarily contains about 15% linoleic acid (18:2n-6) and 1% arachidonic acid (AA) (20:4n-6). Plasma triglycerides, phospholipids, and cholesterol esters also are rich in linoleic acid, and the phospholipids and cholesterol esters contain about 10% AA. These findings suggest that the brain probably can obtain an adequate supply of n-6 PUFA from either the plasma FFA or lipoproteins. By contrast, the plasma ordinarily contains only one-tenth as much n-3 PUFA, and the amounts range from 1% alpha-linolenic acid (18:3n-3) in the plasma FFA to 2% docosahexaenoic acid (22:6n-3, DHA) in the plasma phospholipids. The main n-3 PUFA in the brain is DHA. Therefore, if the plasma FFA is the primary source of fatty acid for the brain, much of the DHA must be synthesized in the brain from n-3 PUFA precursors. Alternatively, if the brain requires large amounts of preformed DHA, the phospholipids contained in plasma lipoproteins are the most likely source.     

Omega-3 fatty acid treatment and T(2) whole brain relaxation times in bipolar disorder

OBJECTIVE: The authors hypothesized that changes in brain membrane composition resulting from omega-3 fatty acid administration in patients with bipolar disorder would result in greater membrane fluidity, as detected by reductions in T(2) values. METHOD: Women with bipolar disorder (N=12) received omega-3 fatty acids for 4 weeks. A cohort of bipolar subjects (N=9) and a group without bipolar disorder (N=12) did not receive omega-3 fatty acids. T(2) values were acquired at baseline and after 4 weeks. RESULTS: Bipolar subjects who received omega-3 fatty acids had significant decreases in T(2). There was a dose-dependent effect when the bipolar omega-3 fatty acid group was subdivided into high- and low-dose cohorts. CONCLUSIONS: Omega-3 fatty acids lowered T(2) values, consistent with the hypothesis that the fluidity of cell membranes was altered. Further studies are needed to clarify the significance of alterations in brain physiology induced by omega-3 fatty acids, as reflected in T(2) values.     

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A₂'s (PLA₂s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.     

The correlation between biochemical and histopathological findings in adrenoleukodystrophy

Nine areas of the brain from a case of adrenoleukodystrophy were examined histopathologically and by gas chromatography for fatty acid content. The main findings were: (1) the degree of demyelination was related to the pentacosanoic and hexacosanoic to docosanoic acid ratios (C25:0/C22:0 and C26:0/C22:0); gliosis was related to the ratios of several fatty acids to docosanoic acid; (2) there was a shift towards smaller components (C22:0, C23:0, C24:0) of the saturated fatty acid series in the less affected areas, to larger components and various minor components in regions of active demyelination; (3) mainly saturated fatty acids of the middle class components (C24:0, C25:0 and C26:0) were found in severely affected areas where the active process is complete. Because a region of high long chain fatty acid content, lacking histopathological change, was detected, the hypothesis is presented that the primary event in childhood ALD is related to defective lipid metabolism and that this preceeds demyelination.     

Effect of postweaning hyperphenylalaninemia on brain development in rats: myelination, lipid, and fatty acid composition of myelin.

We examined the effect of hyperphenylalaninemia induced during the early postweaning period on brain weight, myelin content of brain, and lipid and fatty acid composition of lipids of myelin. Hyperphenylalaninemia was induced in 21-day-old weanling rats by treatment with phenylalanine (Phe) plus p-chlorophenylalanine (PCPA) daily for 30 days. The body weights of control and hyperphenylalaninemic rats did not differ significantly. The brain weight of experimental rats, however, was significantly lower. The reduction in brain weight of Phe + PCPA-treated rats was reflected in lower yields of myelin. The gross composition (proportion of protein and proportion of major lipid components) of myelin from brains of control and experimental rats was not appreciably different. Myelin from hyperphenylalaninemic rat brains, however, contained significantly lower amounts of sulfatide and reduced proportions of unsaturated fatty acids. We conclude that, as in suckling rats, hyperphenylalaninemia induced during the early postweaning period impairs myelination and the myelin synthesized is abnormal with respect to sulfatide content and proportion of unsaturated fatty acids, and as a consequence, the biochemical reactivity and stability of myelin may be altered in hyperphenylalaninemic rat brains.