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

Abstract

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 tnat 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°; 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.