Ascorbic acid deficiency and hepatic UDP-glucuronyl transferase. Qualitative and quantitative differences

The effect of dietary ascorbate on hepatic UDP glucuronyltransferase (UDPGT) appears to be selective in that only certain isozymes of UDPGT are jeopardized. In this study, ascorbic acid deficiency produced a 68% reduction in the specific activity of hepatic UDPGT towards p-nitrophenol. Earlier studies showed a reduction in UDPGT activity towards p-aminophenol in ascorbate-deficient guinea pigs, whereas bilirubin and acetaminophen glucuronidation were unaffected. Kinetic studies suggest that p-aminophenol and p-nitrophenol are metabolized by a single isozyme in that p-nitrophenol was found to be a competitive inhibitor of p-aminophenol glucuronidation. Both qualitative and quantitative studies on partially purified UDPGT from ascorbate-deficient and ascorbate-supplemented guinea pigs were carried out to investigate the biochemical role of the vitamin. Qualitative differences were observed in UDPGT from ascorbate-deficient animals and included an increased lability to: thermal inactivation; storage at 4 degrees; and purification with UDP-glucuronic acid agarose column chromatography. Furthermore, an analysis of the microsomal membrane showed a 14% increase in membrane fluidity in ascorbate deficiency. Ascorbic acid added in vitro could not reverse the increase in fluidity observed in ascorbate-deficient microsomal membranes; however, ascorbylpalmitate, a more lipophilic form of the vitamin, was effective. Palmitic acid had no effect on membrane fluidity in microsomes from either the ascorbate-supplemented or ascorbate-deficient animals. This increase in membrane fluidity could not be explained by differences in cholesterol, total phospholipid, or phosphatidylcholine content of hepatic microsomes. Furthermore, a quantitative reduction in UDPGT partially purified from ascorbate-deficient guinea pigs was indicated by a marked reduction in protein banding at 55,000 daltons when compared to UDPGT partially purified from ascorbate-supplemented animals.     

Ascorbic acid deficiency and the flavin-containing monooxygenase

Activity of the flavin-containing monooxygenase (FMO) was reduced significantly in ascorbic acid deficient guinea pigs. Reduction in oxidation of dimethylaniline (DMA) and of thiobenzamide was associated with a decrease in the activity of the FMO. In both ascorbate supplemented and deficient guinea pig hepatic 12,000 g supernatant fractions, SKF-525A and n-octylamine did not inhibit DMA N-oxidation. Phenobarbital pretreatment did not increase the rate of N-oxidation of DMA. In addition, hepatic supernatant fractions thermally treated at 50 degree were unable to N-oxidize DMA, but 80% of the cytochrome P-450 activity was retained. Also, N-oxidation of DMA was reduced by 53% at pH 7.0, while oxidation of cytochrome P-450 specific substrates was inhibited by only 19%. Kinetic studies of DMA N-oxidation indicate no significant change in the apparent Km in ascorbate supplemented or deficient animals. The in vitro addition of ascorbic acid had no effect on the activity of the FMO. The toxicological implications of the reduction in FMO activity in ascorbic acid deficiency are discussed.     

The inherited deficiency of hepatic UDP-glucuronyltransferase: Structure-activity relationships of in vitro stimulators

More than 18 compounds have been tested for their ability to stimulate defective UDP-glucuronyltransferase activity of Gunn rat liver homogenates towards 2-aminophenol, up to levels of transferase activity in similarly treated Wistar rat liver homogenates. The minimum structural requirements of an effective compound are a combination of the presence of an electron-attracting atom or group and a sparing solubility in water. The activation of defective UDP-glucuronyltransferase towards 2-aminophenol by pentan-3-one is reversible. The possible mechanism of action of alkyl ketone activators is discussed.     

Ascorbic acid deficiency and pituitary adrenocortical activity in the guinea-pig

1. Guinea-pigs kept on a diet deficient in ascorbic acid lost weight and became moribund in about 24 days.2. The adrenal ascorbic acid concentration fell rapidly during the first 2 weeks, and the plasma corticosteroid concentration and 17-oxogenic steroid excretion rose sharply in the third week of ascorbic acid deficiency.3. Both histamine and corticotrophin increased the plasma corticosteroid concentration when injected during the second week but failed to change the pre-existing high concentration of the steroid in the third week of ascorbic acid deficiency.4. The observations confirm that ascorbic acid is not involved in corticoidogenesis and that scurvy is a severe stress which increases adrenocortical activity to such an extent that the rate of synthesis of corticosteroids is incapable of matching the rate of their release.     

Ascorbic acid in thyroidectomized rats. II) Ascorbic acid status of the storage tissues and hepatic biosynthesis of glucuronic acid

Depletion of ascorbic acid from adrenals, brain and epididymis along with loss in weight were noticed in the state of thyroidectomy. This decrease appears to be due to an effect of thyroidectomy on the membrane integrity since the membrane bound sialic acid was found to be significantly lowered in these tissues as a consequence of the elevated activity of sialidase. Thyroidectomy was also found to cause an adverse effect on the activities of hepatic UDP-glucuronyl transferase and beta-glucuronidase with no alteration in UDP-glucose dehydrogenase.     

Ascorbic acid deficiency aggravates stress-induced gastric mucosal lesions in genetically scorbutic ODS rats

We examined whether ascorbic acid (AA) deficiency aggravates water immersion restraint stress (WIRS)-induced gastric mucosal lesions in genetically scorbutic ODS rats. ODS rats received scorbutic diet with either distilled water containing AA (1 g/l) or distilled water for 2 weeks. AA-deficient rats had 12% of gastric mucosal AA content in AA-sufficient rats. AA-deficient rats showed more severe gastric mucosal lesions than AA-sufficient rats at 1, 3 or 6 h after the onset of WIRS, although AA-deficient rats had a slight decrease in gastric mucosal AA content, while AA-sufficient rats had a large decrease in that content. AA-deficient rats had more decreased gastric mucosal nonprotein SH and vitamin E contents and increased gastric mucosal lipid peroxide content than AA-sufficient rats at 1, 3 or 6 h of WIRS. These results indicate that AA deficiency aggravates WIRS-induced gastric mucosal lesions in ODS rats by enhancing oxidative damage in the gastric mucosa. Received 12 July 2006; accepted 21 August 2006     

Ascorbic acid and alcohol oxidation

Methanol and ethanol were rapidly metabolized to formaldehyde and acetaldehyde in the presence of ascorbate, 1,10-phenanthroline and either guinea pig hepatic 100,000 g supernatant or 12,000 g pellet fractions. The specific activity of methanol oxidation was 1720 nmoles formaldehyde formed/min/mg protein in the 100,000 g fraction and 790 in the 12,000 g pellet fraction. The specific activity of ethanol oxidation was 1590 nmoles acetaldehyde formed/min/mg protein in the 100,000 g fraction and 820 in the 12,000 g pellet fraction. The activity was enzymatic in that it was linear with time, proportional to protein concentration, and sensitive to temperature. Catalase appeared to be the enzymatic component responsible for the oxidation. In this ascorbate-dependent alcohol oxidation system, oxygen was consumed and H2O2 was formed. When purified catalase and ascorbate were used, complex I was detected and methanol was oxidized.     

Ascorbic acid and performance in man

Effects on performance of 1,2 and 4g ascorbic acid were studied from 0.5–5.5 h after ingestion in six healthy females. Diazepam (5 mg) was included as an active control, and it impaired digit symbol substitution, visuomotor coordination and complex reaction time. There were no effects of any dose of ascorbic acid on performance.     

Ascorbic acid and chemically induced methemoglobinemias

Sodium nitrite was added to suspensions of washed guinea pig red cells in concentrations which converted about a third of the total blood pigment to methemoglobin. Various aliquots also contained added ascorbic acid in concentrations of 0.05, 0.5, or 5.0 mm (“physiological” levels are between 0.05 and 0.1 mm). Methemoglobin levels were followed for 2 hr at 37°C. None of the tested concentrations of ascorbate significantly attenuated methemoglobin formation. On the contrary, significantly higher methemoglobin levels were sometimes found with 5.0 mm ascorbate. Guinea pigs were maintained several weeks on a scorbutogenic diet with or without ascorbate supplementation in their water. When blood ascorbate levels were significantly different, both groups were given intraarterial sodium nitrite, 40 mg/kg, and methemoglobin levels were followed with time as above. At none of the sampling times was there a significant difference in the methemoglobin levels between the 2 groups. As judged by the appearance of radioactivity in blood at various times after sc injection, the two groups also showed no difference in the rates at which they absorbed KS¹⁴CN. In human red cell suspensions 5.0 mm ascorbate significantly attenuated the methemoglobinemic response to tested concentrations of sodium nitrite, hydroxylamine and phenylhydroxylamine at 1 hr, but lower concentrations of ascorbate were without effect. When human red cells were first exposed to nitrite, then washed and ascorbate added, significant acceleration of methemoglobin reduction was observed with 5.0 mm ascorbate at 12, 24, and 36 hr, and with 0.5 mm ascorbate at 12 and 24 hr. In contrast, the oxygen capacities of suspensions with 5.0 mm ascorbate were significantly higher than control only at 12 and 24 hr, whereas with 0.5 mm ascorbate they were significantly higher at 12, 24, and 36 hr. In plain buffer, 5.0 mm ascorbate produced a small but significant loss of the tested concentration of sodium nitrite within 2 hr, but not at shorter intervals or at lower ascorbate concentrations.     

Effects of diethofencarb on thyroid function and hepatic UDP-glucuronyltransferase activity in rats.

To examine the mechanism and toxicological significance of thyroidal tumor observed slightly in a long-term rat study with diethofencarb (isopropyl 3,4-diethoxycarbanilate), male Sprague-Dawley rats were fed diethofencarb in diets at concentrations of 0, 5,000 or 20,000 ppm for 3 months. Examinations mainly for thyroid functions including thyroid uptake of 125I, serum thyroid hormone and thyroid stimulating hormone (TSH) level, hepatic UDP-glucuronyltransferase (UDP-GT) activity and histopathological examination in thyroid were performed at week 13. Decreases of body weights and food consumptions were observed at and above 5,000 ppm. Under these conditions, decrease of serum free T4 and increase of serum TSH level were observed only at 20,000 ppm, concurrently with liver weight increase at and above 5,000 ppm and increase of hepatic UDP-GT activity at 20,000 ppm. However, no compound related effects were noted in thyroid weight, thyroid uptake of 125I and gross or histopathological examination in thyroid. These results indicate that the administration of diethofencarb leads to an increase in UDP-GT activity and acceleration of thyroid hormone excretion from the liver. The acceleration causes a decrease in serum free T4 level, triggering the feedback mechanism of the pituitary gland, promotion of TSH release and consequently an increase in serum TSH level. Thus, the slightly higher incidence of thyroid follicular cell tumors observed in the chronic and oncogenicity study with non-genotoxic diethofencarb is considered to be caused by these weak pituitary-thyroid hormonal imbalances. The toxicological significance in humans is extremely low according to the well established facts that the chronic TSH stimulating would not induce thyroid tumors in humans and humans may be less sensitive than rats in regard to the response to goitrogenic stimuli.     

Ascorbic acid deficiency decreases specific forms of cytochrome P-450 in liver microsomes of guinea pigs

Ascorbic acid (VC) deficiency resulted in a decrease in the activities of aminopyrine N-demethylase, aniline hydroxylase, and p-nitroanisole O-demethylase and in the content of cytochrome P-450, as spectrally determined, whereas it caused an increase in the activities of 6 beta-hydroxylases for testosterone and progesterone in liver microsomes of guinea pigs. Western blot analysis of liver microsomes with antibodies to rat P-448-H (P-4501A2), P-450j (P-450IIE), P-450 PB-1 (P-450IIIA), and P-450b (P-450IIB1) showed that VC deficiency decreased the amount of cytochrome P-450 immunochemically related to P-450IA2 and P-450IIE but did not change the amount of the form that was cross-reactive with antibodies to P-450IIB1 and tended to slightly increase (not statistically significantly) the amount of the form of the cytochrome immunochemically related to P-450IIIA. The larger decrease by VC deficiency in the amount of cytochrome P-450 that was cross-reactive to the rat P-450IA2 resulted in a lower capacity of liver microsomes to activate promutagens, such as 2-amino-3-methyl-imidazo(4,5-f)quinoline and aflatoxin B1. These results indicate that VC deficiency in guinea pigs differentially affects the content of individual forms of cytochrome P-450.     

Ascorbic acid and pyridoxine in experimental anaphylaxis

Two vitamins, ascorbic acid (AA) and pyridoxine have been suggested by others as useful drugs for the treatment of bronchial asthma, although the views concerning AA or controversial. We have tested both vitamins in some models of histamine release and experimental anaphylaxis. AA does not inhibit mast cell degranulation induced by phospholipase A and histamine release from isolated rat mast cells induced by compound 48/80 or antigen (egg albumin). On the contrary, in the latter tests pyridoxine exerts inhibition in a range of concentrations from 10(-3)-10(-2) M. We conclude: 1. There is no experimental basis for considering ascorbic acid as a prophylactic antiasthmatic drug as is disodium cromoglycate. 2. Pyridoxine must receive additional basic and clinical investigations in this field.     

Ascorbic acid, dehydroascorbic acid and glutathione in liver disease

Controlled studies were conducted to find out the plasma values of ascorbic acid, dehydroascorbic acid (DHA), urinary excretion of ascorbic acid and blood levels of glutathione in patients with viral hepatitis, alcoholic hepatitis, cirrhosis of liver and carcinoma of liver. Leucocyte ascorbic acid and DHA/AA index were also determined in order to assess the ascorbic acid status of these patients. It was observed that the plasma and leucocytes contents of ascorbic acid were significantly subnormal with markedly decreased urinary excretion in these patients. Decreased level of glutathione and significantly higher level of DHA reflect an over all reducing status of the body is markedly deranged in these conditions. Further it was observed that the DHA/AA ratios were significantly altered in these groups of patients.     

Ascorbic acid stability in aqueous solutions

Different water purity provokes a great variation of the stability of ascorbic acid and isoascorbic acid solutions. The effect of temperature on ascorbate aerobic oxidation was assessed by means of Arrhenius plots from which thermodynamic parameters were derived. The presence of bovine serum albumin drastically reduces the vitamin oxidation rate regardless of stereoisomerism. On the other hand the interaction with alkaline phosphatase, an enzyme inhibited by preincubation with vitamin C, does not modify significantly the stability in the experimental conditions used.