Ascorbic acid deficiency and hepatic UDP-glucuronyltransferase.
The effect of dietary ascorbic acid on hepatic microsomal UDP-glucuronyltransferase (UDPGT) activity towards p-aminophenol, bilirubin, and acetaminophen was investigated. Ascorbate deficiency produced a 33% reduction in the specific activity of UDPGT towards p-aminophenol, whereas there was no difference between microsomes from ascorbate-deficient and supplemented guinea pigs in the activity towards bilirubin and acetaminophen. This suggests that the effect of the vitamin is on a specific isozyme. This reduction was correlated with the reduced quantity of hepatic microsomal cytochrome P-450, which has been previously reported for ascorbate-deficient guinea pigs. No difference was found in the apparent affinity for the substrate, p-aminophenol, or the cofactor, UDP-glucuronic acid. Differences in microsomal UDPGT activity towards p-aminophenol occurred between the two groups with membrane-perturbing processes such as sonication and Triton X-100. Sonication and magnesium chloride were found to increase activity 329% in ascorbate-supplemented animals and 138% in the ascorbate-deficient group. The addition of ascorbate acid in vitro, or its analog d-isoascorbic acid, could protect against the detrimental effects of excess substrate by maintaining a linear enzymatic rate over a 30-min time period; there was no significant effect on the initial rate of hepatic microsomal UDPGT activity in the ascorbate-supplemented animals whereas there was a significant increase in the ascorbate-deficient group. Glutathione was as effective as ascorbic acid in protecting against the detrimental effects of excess substrate whereas cysteine and dimethyltetrapteridine were only partially effective. Ascorbyl-2-sulfate and alpha-tocopherol had no significant effect.(ABSTRACT TRUNCATED AT 250 WORDS)
D000082:Acetaminophen / Q000378:metabolism; D000627:Aminophenols / Q000378:metabolism; D000818:Animals; D001205:Ascorbic Acid / Q000008:administration & dosage; D001206:Ascorbic Acid Deficiency / Q000201:enzymology* / Q000378:metabolism; D001663:Bilirubin / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004032:Diet; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006168:Guinea Pigs; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D008954:Models, Biological
D000627:Aminophenols; D000082:Acetaminophen; D003577:Cytochrome P-450 Enzyme System; D014453:Glucuronosyltransferase; D001205:Ascorbic Acid; D001663:Bilirubin