Availability of 4'galactosyllactose (O-beta-D-galactopyranosyl-(1----4)-O-beta-D-galactopyranosyl-(1----4)- D-glucopyranose) in rat

O-beta-D-Galactopyranosyl-(1----4)-O-beta-D-galactopyranosyl-(1----4)-D- glucopyranose (designated as 4'GL) is produced from lactose by Cryptococcus laurentii. The influence of chronic ingestion of 4'GL on body weight gain, organ weight, serum lipids, and liver lipids was investigated in rats. The body weight gains of the 5% and 10% 4'GL-diet groups were higher than that of the control group. Food intake and fecal dry weight were significantly increased (p less than 0.05) by 4'GL feeding. The 4'GL diet produced a significant increase (p less than 0.01) in the wet weight and contents of both the cecum and the colon. However, no significant increase was observed in the weight of the stomach, small intestine, liver, or other organs. The effects of 4'GL on serum and liver lipid levels were not observed in this experiment. The digestion of 4'GL was measured in vitro using the artificial gastric juice, alpha-amylase of human saliva, alpha-amylase of hog pancreas, and mucosa of rat intestine. 4'GL was not hydrolyzed by these enzymes. Long-term ingestion of 4'GL did not cause any induction of 4'GL hydrolyzing enzyme activity in the rat small intestine.     

Utilization and excretion of a new sweetener, fructooligosaccharide (Neosugar), in rats

In order to study the digestibility of the fructooligosaccharide "Neosugar," [U-14C]Neosugar or [U-14C]sucrose was orally administered to germfree, conventional and antibiotic-treated rats and the radioactivities of expired 14CO2, urine and feces were determined 24 h later. More than 50% of the Neosugar was expired as CO2 in conventional rats. This was the same as for sucrose, but the time course was delayed by about 2 h. In germfree rats, no 14CO2 was released for the first 8 h, and 14CO2 released after 8 h probably reflected bacterial colonization of the gut. The radioactivity of the urine was about 3-4% in all groups, but that of the feces from germfree rats was about eight times higher than the level in conventional rats. When [U-14C]Neosugar was anaerobically incubated with the cecal contents of conventional rats, more than 10% of the added Neosugar was metabolized to CO2, about 66% to volatile fatty acids and about 7% to microbes. More than 58% of 1-14C-volatile fatty acids such as acetic acid, propionic acid or butyric acid injected directly into the cecum of conventional rats was excreted as CO2 within 24 h. These results indicate that Neosugar given orally to rats is metabolized mainly to volatile fatty acids and CO2 by intestinal microorganisms, and the volatile fatty acids produced are absorbed and further converted to CO2 in the body. Thus, the data indicate that Neosugar is partially utilized as an energy source.     

Metabolism of maltitol by conventional rats and mice and germ-free mice, and comparative digestibility between maltitol and sorbitol in germ-free mice

The metabolism of maltitol (4-alpha-D-glucosylsorbitol) was assessed in fasting conventional (C) rats, C mice and germ-free (GF) mice, using [U-14C]maltitol. The radiorespirometric patterns of 14CO2 collected for 48 h after the administration of labelled maltitol were characterized by a constant rate of 14CO2 production lasting 4 h for both C rats and mice. The pattern for the GF mice showed a peak at the second hour followed immediately by a slow decrease. The percentage recovery of 14CO2 was significantly lower for the GF mice (59%) compared with C animals (72-74%). Urine, faeces and intestinal contents after 48 h totalled 19% of the administered radioactivity in the C rats and mice and 39% in the GF mice. The digestibility of maltitol and the absorption of sorbitol in GF mice was also assessed. The caecum and small intestine of GF mice, 3 h after administration of equimolar quantities of maltitol (140 mg/kg body-weight) or sorbitol (70 mg/kg body-weight), contained 39 and 51% of the ingested dose respectively, present mostly in the caecum as sorbitol. The alpha-glucosidase (maltase) (EC3.2.1.20) activity of the small intestine was appreciably higher (1.5-1.7 times) in the GF mice than in the C mice. These results suggest that the enzymic activities in the small intestine of mice and rats are sufficient to hydrolyse maltitol extensively. Consequently, the slow absorption of sorbitol seems to be an important factor limiting the overall assimilation of maltitol in the small intestine.     

Utilization of orally administered D-[14C]mannitol via fermentation by intestinal microbes in rats

To investigate the available energy of orally administered [(14)C]mannitol via intestinal microbes, [(14)C]mannitol (222 kBq, 105 mg) or [(14)C]glucose (222 kBq, 105 mg) was administered to conventional rats and antibiotics-treated rats whose intestinal microbes were depleted by drinking water containing antibiotics, respectively. The exhausted CO(2), feces and urine were then separately collected at 2, 4, 6, 8, 10, 12 and 24 h after administration of the test solution. In the conventional rats, 45% of administered radioactivity was recovered as (14)CO(2) in the administration of [(14)C]mannitol, while 57% of administered radioactivity was recovered as (14)CO(2) following the administration of [(14)C]glucose for 24 h. The time sequence for the (14)CO(2) excretion from [(14)C]mannitol was delayed as compared to [(14)C]glucose by about 4-6 h (p<0.05). However, when [(14)C]mannitol was orally administered to antibiotics-treated rats, only 3% of administered radioactivity was excreted as (14)CO(2) for 24 h. The total radioactivity of the gastrointestinal contents and feces for 24 h after administration was over 70%, much higher than those of the conventional rats (p<0.05). When a half dose (222 kBq, 52.5 mg) of [(14)C]mannitol was administered to conventional rats, the recovery as (14)CO(2) for 24 h (%) was significantly higher than that of a regular dose of [(14)C]mannitol (105 mg). When cold mannitol (105 mg) was orally administered to the antibiotics-treated rats, about 9% of intact mannitol was excreted in feces within 48 h after administration. However, no intact mannitol was detected in the conventional rats. These results demonstrate that more than 95% of mannitol administered orally is utilized via fermentation by intestinal microbes.     

Comparative metabolism of L-methionine, DL-methionine and DL-2-hydroxy 4-methylthiobutanoic acid by broiler chicks

1. Metabolism, in broiler chicks, of DL-2-hydroxy 4-methylthiobutanoic acid (DL-HMB), DL-methionine and L-methionine was compared in vivo using 14C-labelled tracers. 2. The distribution of L-[1-14C]methionine and DL-[1-14C]HMB in the major body tissues was examined for a period of 120 min after administration. 3. The relative oxidation (14CO2 exhaled), excretion and incorporation into tissue protein of L-[1-14C]methionine, DL-[1-14C]methionine and DL-[1-14C]HMB were measured in fed birds. 4. Tissue distribution of L-[1-14C]methionine and DL-[1-14C]HMB differed during 60-90 min following administration. 5. The production of 14CO2 from each of the tracers was similar but excretion of 14C-labelled material was very different with the greatest excretion from DL-[1-14C]HMB and the least from L-[1-14C]methionine. 6. The incorporation of 14C into tissue proteins varied with the tracer given and the tissue examined. Liver and kidney had equivalent incorporation from each of the tracers while other tissues examined showed lower incorporation from DL-[1-14C]methionine and DL-[1-14C]HMB. 7. The results show that DL-HMB, D-methionine and L-methionine are metabolized differently in vivo and that they are excreted in differing proportions. There is also a difference in the ability of each to act as a precursor for protein synthesis in tissues other than liver.     

Metabolism and disposition of erythritol after oral administration to rats.

The metabolism and disposition of erythritol was studied using [14C]erythritol in rats. When [14C]erythritol was administered orally at a dose of 0.1 g/kg body wt to male rats, only 6% of the total radioactivity was excreted as expired 14CO2 and 88% was excreted in the urine within 24 h. The excreted metabolite in the urine consisted of a single component identified as intact [14C]erythritol. The excretion of 14CO2 and the incorporation ratios of radioactivity into tissues increased with the oral dosage. After rats were given an intravenous injection of [14C]erythritol, approximately 1% was excreted as 14CO2 and greater than 94% was excreted in the urine as intact [14C]erythritol. The excretion of 14CO2 within 24 h was increased to approximately 10% when [14C]erythritol was administered to rats that had been adapted to erythritol by feeding a diet containing 10% erythritol for 2 wk. When [14C]erythritol was incubated in vitro with the cecal contents from rats adapted to erythritol, greater than 20% was fermented to 14CO2 and 60% to short-chain fatty acids in 6 h. These results indicate that most orally administered erythritol was excreted in the urine without any degradation and that the remainder was transferred to the lower intestine and fermented by microbes.     

Tryptophan metabolism in vitamin B6-deficient mice

Vitamin B6 deficiency was induced in mice by maintenance for 4 weeks on a vitamin B6-free diet. Tryptophan metabolism was assessed by determining the urinary excretion of tryptophan metabolites, the metabolism of [14C]tryptophan in vivo and the formation of tryptophan and niacin metabolites by isolated hepatocytes. The vitamin B6-deficient animals excreted more xanthurenic acid and 3-hydroxykynurenine, and less of the niacin metabolites N1-methyl nicotinamide and methyl-2-pyridone-4-carboxamide, than did control animals maintained on the same diet supplemented with 5 mg vitamin B6/kg. After intraperitoneal injection of [14C]tryptophan, vitamin B6-deficient mice showed lower liberation of 14CO2 from [methylene-14C]tryptophan and [U-14C]tryptophan than did controls, indicating impairment of kynureninase (EC 3.7.1.3) activity. There was no difference between the two groups of animals in the metabolism of [ring-2-14C]tryptophan. Hepatocytes isolated from the vitamin B6-deficient animals formed more 3-hydroxykynurenine and xanthurenic acid than did cells from control animals, but also formed more NADP and free niacin.     

Bioavailability of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) to the prairie vole (Microtus ochrogaster)

Concerns have been raised over potential bioavailability and biotransfer of energetic materials such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The present study assessed plant-incorporated [14C]RDX and plant-derived [14C]RDX-metabolite ingestion by the prairie vole (Microtus ochrogaster). The animals were fed labeled chow (maximum, < or =10 g/d) for 5 or 7 d, followed by a 6- or 4-d chase period. More than 95% of all label presented was recovered in the summed excreta, with 74% of this in the fecal nonabsorbed bulk. Greater than 20% of the presented [14C]RDX and plant-derived [14C]RDX metabolites were absorbed by the animals' digestive tracts. These materials were either metabolized to (14)CO2 (8-10% of the total label) or removed as nitrogenous waste through the kidneys (10-14%). Both 14C-urine and (14)CO2 excretion continued after the feces cleared, indicating ongoing metabolism of the labeled material. Approximately 4% was retained within the tissues at death, with the highest total activity in the liver and the highest specific activity in the testes. Other labeled tissues included the lung, heart, brain, spleen, skeletal muscle, bone, and pancreas. All these tissues containing [14C]RDX-derived materials are available to subsequent predators, indicating a potential for transfer to a higher trophic level.     

Free amino acid patterns and the distribution of 14C from [U-14C]-L-Leucine, [U-14C]-L-Lysine, and [U-14C]-L-Alanine in the tissues of young adult rats.

In studies designed to evaluate the role of the liver and muscle in modifying the plasma amino acid response, three levels of dietary amino acids, 3.6%, 4.8% and 6.0% were fed to three groups of young adult male rats for 2 weeks. After fasting, either [U-14C]-L-leucine, [U-14C]-L-lysine or [U-14C]-L-alanine was administered intragastrically with a portion of diet. After a 4.5 hour fast, rats were killed. Distribution of radioactivity was evaluated in trichloroacetic acid (TCA) supernatants and lipid-extracted precipitates of plasma, liver and gastrocnemius muscle, and in expired CO2. Free amino acids were measured and specific activities of amino acids were determined. Rats fed 6.0% or 4.8% amino acids lost less weight than those fed 3.6%. A high percentage of radioactivity from 14C-leucine and 14C-lysine was recovered in the lipid-extracted precipitate. TCA supernatants from rats fed 14C-leucine contained low levels of radioactivity. A large percentage (60%) of the radioactivity from 14C-alanine was expired in 14CO2. The percentages of alanine found in TCA precipitates were very low. Significant increases in concentration associated with increments in dietary amino acids occurred in plasma free isoleucine, leucine, valine and tyrosine; liver free histidine; and muscle free leucine and threonine. Concentrations of many amino acids were depressed in the muscle of rats fed 4.8% amino acids whereas they increased in response to 6.0%. Protein and free amino acid specific activities indicated no change in rates of protein synthesis. Data from this type of experiment may assist in interpreting the role of the liver and muscle in modifying the plasma amino acid response to dietary amino acids.     

Intestinal glucose absorption but not endogenous glucose production differs between colostrum- and formula-fed neonatal calves

Glucose supply markedly changes during the transition to extrauterine life. In this study, we investigated diet effects on glucose metabolism in neonatal calves. Calves were fed colostrum (C; n = 7) or milk-based formula (F; n = 7) with similar nutrient content up to d 4 of life. Blood plasma samples were taken daily before feeding and 2 h after feeding on d 4 to measure glucose, lactate, nonesterified fatty acids, protein, urea, insulin, glucagon, and cortisol concentrations. On d 2, additional blood samples were taken to measure glucose first-pass uptake (FPU) and turnover by oral [U-(13)C]-glucose and i.v. [6,6-(2)H(2)]-glucose infusion. On d 3, endogenous glucose production and gluconeogenesis were determined by i.v. [U-(13)C]-glucose and oral deuterated water administration after overnight feed deprivation. Liver tissue was obtained 2 h after feeding on d 4 and glycogen concentration and activities and mRNA abundance of gluconeogenic enzymes were measured. Plasma glucose and protein concentrations and hepatic glycogen concentration were higher (P < 0.05), whereas plasma urea, glucagon, and cortisol (d 2) concentrations as well as hepatic pyruvate carboxylase mRNA level and activity were lower (P < 0.05) in group C than in group F. Orally administered [U-(13)C]-glucose in blood was higher (P < 0.05) but FPU tended to be lower (P < 0.1) in group C than in group F. The improved glucose status in group C resulted from enhanced oral glucose absorption. Metabolic and endocrine changes pointed to elevated amino acid degradation in group F, presumably to provide substrates to meet energy requirements and to compensate for impaired oral glucose uptake.     

Effects of chronic ethanol feeding on the metabolism of tryptophan and nicotinamide in rats

Oxidation of tryptophan and urinary excretion of N1-methylnicotinamide were studied in rats fed a modified DeCarli-Lieber liquid diet of low (1.1 mg/liter or 5 mg/kg dry diet) and adequate (5.5 mg/liter or 25 mg/kg dry diet) nicotinamide content with and without ethanol replacing carbohydrate to 36% of total calories. Protein was restricted to 12% of calories so that the tryptophan content was about 0.16% of dry weight. Rats were fed for 30 or 40 days and feeding was restricted to 7 or 12 hours per day. N1-Methylnicotinamide was measured in urine collected during the last 2 days of feeding. At the end of the experimental period, fasted rats were injected intraperitoneally with 0.8 muCi of L-[ring-2-14C]tryptophan (0.12 or 0.11 muCi/mg tryptophan) or 0.7 muCi of [14C]formate (0.44 muCi/mg formate) per 100 g body weight. Expired CO2 was collected every 30 minutes or hourly for 5 or 6 hours. N1-Methylnicotinamide excretion was increased in ethanol-fed rats (P less than 0.05). The percent of injected [14C]tryptophan recovered as 14CO2 was increased in rats fed ethanol in a low niacin diet as compared to controls (P less than 0.01). Recovery of 14CO2 from [14C]formate oxidation was not affected by ethanol feeding. These findings suggest that the initial oxidation of tryptophan by tryptophan 2,3-dioxygenase is enhanced following chronic ethanol feeding.     

Recovery of labeled CO2 during the infusion of C-1- vs C-2-labeled acetate: implications for tracer studies of substrate oxidation

In this study we determined the rate of conversion of carbon-labeled acetate to carbon dioxide in normal human volunteers and in anesthetized dogs. In human subjects (n = 4), [1-13C]acetate was infused on one occasion, and [2-13C]acetate was infused in the repeat study in the same subjects. In postabsorptive volunteers (n = 6), 81.2 +/- 6.5% (mean +/- SEM) and 53.1 +/- 7.4% of infused 13C was recovered as 13CO2 when [1-13C]- or [2-13C]acetate, respectively, were infused. In one subject studied during exogenous glucose infusion at 3.5 mg.kg-1.min-1, recovery of 13CO2 was 72.7% and 38.5% from [1-13C]- and [2-13C]acetate, respectively. In dogs, [1-14C]- and [2-13C]acetate were infused simultaneously. Recovery of 14CO2 was 75.9 +/- 2.5% of infused isotope whereas recovery of 13CO2 was 40.8 +/- 1.9%. We concluded that the position of the label in acetyl coenzyme A (CoA) determines the extent to which the process of oxidation of labeled acetyl CoA is reflected in labeled carbon dioxide excretion.     

Role of intestinal microflora in the metabolism of vitamin B-6 and 4'-deoxypyridoxine examined using germfree guinea pigs and rats

In previous work identification of urinary metabolites of 4'-deoxypyridoxine which had been oxidized in the 5'-position and long-term dilution of labeled urinary metabolites with unlabeled molecules suggested possible microbial contributions. In the current studies germfree guinea pigs were able to convert 4'-deoxypyridoxine to 4'-deoxy-5-pyridoxic acid demonstrating that the ability to oxidize the 5'-position is not restricted to microorganisms. Labelling curves for urinary pyridoxic acid in rats continuously fed [14C]pyridoxine since weaning were similar in conventional and germfree animals indicating that any vitamin B-6 synthesized in the intestinal tract was not readily absorbed and metabolized. Therefore, coprophagy did not make a detectable contribution to vitamin B-6 metabolism in rats receiving a nutritionally complete diet. The difficulty in achieving comparable labeling in adult animals is probably due to very slow turnover of portions of the vitamin B-6 pool and not to microbial production of vitamin B-6. The total pool calculated from the radioactivity in the germ-free rats averaged 16.2 +/- 0.8 nmol vitamin B-6 compounds/g body wt. Only 10% of the ingested label was recovered in the feces. In addition, only about 50% of the label excreted in the urine appeared as 4-pyridoxic acid in rats. These observations suggest that it may be difficult to quantitate the total urinary and fecal excretion of ingested vitamin B-6 without using tracers.     

Effect of nicotinamide intake on urinary excretion of N1-methylnicotinamide and oxidation of [7a-14C]tryptophan in the rat

Oxidation of tryptophan and urinary excretion of N1-methylnicotinamide were studied in weanling rats fed ad libitum for 10 days 12% casein diets containing from 0 to 500 mg of nicotinamide per kilogram of diet. N1-methylnicotinamide was measured in urine collected during the last 3 days of the experiment. After the feeding period, rats fed the niacin-free and 500 mg nicotinamide diets were placed in metabolic cages, and each rat was given 4 g (dry weight) of diet containing a tracer dose of DL-[7a-14C]tryptophan. Expired CO2 was collected hourly for 10 hours. From rats consuming levels of nicotinamide ranging between 20 and 150 mg/kg of diet, N1-methylnicotinamide excretion increased linearly with increasing nicotinamide intake. From these results and others from this laboratory (4), it was calculated that an additional 10--11 mg of dietary tryptophan above the approximately 20 mg/day essential for growth are needed by the growing rat for the formation of 1 mg of nicotinamide. The addition of 500 mg of nicotinamide per kilogram of diet to a niacin-free diet had no effect on the oxidation of DL-[7a-14C]tryptophan.     

Species difference in the disposition of acrylonitrile: quantitative whole-body autoradiographic study in rats and mice

Previous studies from this and other laboratories have indicated the role of species difference in acrylonitrile (VCN) toxicity and its metabolism to cyanide. Our recent studies also indicated a more pronounced elimination of VCN following oral as compared to i.v. administration. To further characterize the mechanism of these differences on the distribution of VCN, quantitative whole-body autoradiographic distribution and elimination studies were conducted at various time points (0.08, 8, 24, 48 h) following the administration of an equivalent i.v. dose of 2-[14C]-VCN to male Fischer rats and male CD-1 mice. Whole-body autoradiographs obtained from freeze-dried and acid-extracted sections of rats and mice demonstrated a rapid uptake of 14C in liver, lungs, spleen and bone marrow at early time intervals. Quantitatively, the uptake, retention and covalent interaction of 14C were higher in organs of rats as compared to mice, over 48 h. Mice eliminated 74% of the total administered dose of 2-[14C]-VCN (expired air 4%, urine 16% and feces 54%), while rats eliminated only 26% of the dose (expired air 2%, urine 4% and feces 20%). Species differences in VCN toxicity seem to be correlated with its rate of elimination. The distribution and elimination data demonstrated that mice divest VCN more rapidly than rats. The study also demonstrated that administration of VCN in rats resulted in covalent interactions and retention of 2-[14C]-VCN/metabolites in the tissues thus exerting more chronic toxicity to rats than to mice.     

Metabolic fate of ingested [14C]-maltitol in man.

Six healthy male subjects were given a single oral dose of 0.4 g of maltitol per kg body weight containing [U-14C]-maltitol (50 microCi) and their breath, urine, feces and blood were collected at appropriate intervals for 48 h to measure the recovery of administered radioactivity. Further, to measure breath hydrogen, 15 healthy male subjects ingested 30 g of maltitol and samples of their breath were collected for a 10-h period. The expired 14CO2 showed a wide peak about 3 h after ingestion and 56% of the administered radioactivity was recovered in 48 h. An additional 0.2% of administered radioactivity was recovered as expired 14CH4, 2.6% in urine and 14.3% in feces, respectively. The obvious increase of breath hydrogen was detected at 1 h after maltitol ingestion and then a big peak at 3.5 h, whereas maltose ingestion did not increase breath hydrogen. Both excretion profiles of breath 14CO2 and hydrogen coincided well. These results demonstrate that a greater part of ingested maltitol is fermented by intestinal microbes than is hydrolyzed by digestive enzymes. Although maltitol is catabolized to carbon dioxide via intestinal microbes, the available energy is much lower than that of digestible sugars such as sucrose.     

Uptake of intact TPGS (d-alpha-tocopheryl polyethylene glycol 1000 succinate) a water-miscible form of vitamin E by human cells in vitro

The mechanism by which TPGS (alpha-tocopheryl succinate esterified to polyethylene glycol 1000 [PEG 1000]) delivers tocopherol (vitamin E) was studied in human fibroblasts and erythrocytes and a human intestinal cell line, Caco-2. The total cellular tocopherol content of saponified samples of fibroblasts or Caco-2 incubated for 4 h with TPGS (4 mumol/L) increased 10-fold without an increase in the free tocopherol content of nonsaponified samples. A 24-h incubation resulted in a free tocopherol content of approximately 20%, suggesting that intracellular hydrolysis of ester bonds had occurred. The increase in total tocopherol content after a 4-h incubation with TPGS was temperature dependent; no change was measurable at 4 degrees C. Addition of metabolic inhibitors during incubation with TPGS at 37 degrees C did not prevent the increase. [14C]TPGS (synthesized from [14C]PEG 1000) was taken up by Caco-2 cells but [14C]PEG 1000 was not. The intracellular total tocopherol (pmol) equaled the [14C]TPGS (pmol), unequivocally demonstrating uptake of the intact TPGS molecule.     

The metabolism of [14C]beta-carotene and the presence of other carotenoids in rats and monkeys

The metabolism of beta-carotene has been studied in both rats and Rhesus monkeys, following the oral administration of [14C]beta-carotene in olive oil supplemented with 1 mg/mL alpha-tocopherol. In the rats, peak serum accumulation of [14C]retinol occurred 4 h after a single oral dose, but we were not able to detect [14C]beta-carotene in rat sera at any time up to 72 h after dosing. Small amounts of [14C]beta-carotene were found in the livers, although 88-94% of the recovered radioactivity was localized in the retinol fraction after saponification. Although radioactivity was also found in fractions other than beta-carotene and retinol, the amounts were too small to allow characterization. In the monkeys, peak accumulation of [14C]retinol in serum occurred between 8 and 24 h after supplementation. Some [14C]beta-carotene was also present. Most of the absorbed radioactivity was stored in the liver as [14C]retinol, although 2-8% was present as [14C]beta-carotene. Other organs also contained [14C]beta-carotene, confirming the ability of the monkey to absorb intact beta-carotene. In addition, monkey livers and other organs were found to contain lutein, zeaxanthin, alpha-cryptoxanthin, beta-cryptoxanthin and beta-carotene, presumably arising from dietary sources.     

Relative importance of the two major pathways for the conversion of cysteine to glucose in the perfused rat liver.

The effects of dietary treatments and substrate availability on the rate of gluconeogenesis from L-cysteine has been investigated in the perfused rat liver. At an optimal concentration (10 mM) of [U-14C]cysteine, after 40 minutes, 3.9% of the label appeared in glucose. This corresponded to 90% of the net glucose coming from cysteine. Cysteine was then shown to be converted to glucose at a physiological concentration of substrate (0.1 mM) as well as at the optimal concentration. After 40 minutes of perfusion with 0.1 mM [U-14C]cysteine as the substrate, livers of 72-hour starved rats incorporated 1.7% of the label into glucose, and livers of rats perfused without prior starvation incorporated 0.53% of the label into glucose. This suggested that cysteine was glucogenic at optimal and physiological concentrations of cysteine in both fed and starved rats. To determine which, if either, of the two suggested pathways for the conversion of cysteine to glucose was quantitatively more important, livers were perfused with [U-14C]cysteine alone or with [U-14C]cysteine plus cysteine sulfinic acid. The addition of cysteine sulfinate (10 mM) reduced the incorporation of 14C from cysteine into glucose from 3.9 to 2.7%. This suggested that one-third of the cysteine to glucose proceeded via the cysteine sulfinate-dependent pathway.     

Distribution of [14C]canthaxanthin and [14C]lycopene in rats and monkeys

The absorption and distribution of [14C]-canthaxanthin and [14C]lycopene were studied in rats and in rhesus monkeys following the oral administration of [14C]canthaxanthin or [14C]lycopene in olive oil supplemented with 1 mg alpha-tocopherol/mL. For canthaxanthin and lycopene, peak accumulation of radioactivity in plasma occurred between 4 and 8 h in rats and between 8 and 48 h in monkeys. In rats, the liver contained the largest amount of both kinds of radioactive pigments. In monkeys, with the exception of one stomach sample, liver was also the major depot organ for both canthaxanthin and lycopene. The other organs tested accumulated various amounts of pigment. No labeled metabolic products of either canthaxanthin or lycopene were found.