Ascorbic acid deficiency and hepatic UDP-glucuronyl transferase.

Abstract

The objective of this investigation was to determine the effect of dietary ascorbic acid on the important hepatic detoxifying enzyme, UDP-glucuronyltransferase. There was a reduction in activity towards p-aminophenol (33-46%) and p-nitrophenol (68%) while bilirubin and acetaminophen glucuronidation were unaffected. p-Nitrophenol was a competitive inhibitor of p-aminophenol glucuronidation while bilirubin was noncompetitive and acetaminophen uncompetitive, suggesting one isozyme is jeopardized in ascorbate-deficient guinea pigs. The reduction in p-aminophenol glucuronidation was correlated with the reduced quantity of hepatic microsomal cytochrome P-450. Qualitative differences were investigated between the ascorbate-deficient and ascorbate-supplemented animals. No differences were found in the apparent affinity (Km) for p-aminophenol or UDP-glucuronic acid. The addition of ascorbic acid in vitro could not restore UDPGT activity in ascorbate-deficient guinea pigs but it protected against the inhibition by excess substrate. D-isoascorbic acid and glutathione were also effective in protecting against the inhibition by excess substrate while ascorbate-2-sulfate was ineffective, indicating that the ene-diol reducing moiety in the vitamin is essential for protection. Preincubating the enzyme with phosphatidylcholine had no effect on UDPGT activity in the ascorbate-supplemented group while activity increased 34% in the ascorbate-deficient group. Partially purified UDPGT from ascorbate-deficient animals had increased liability to thermal inactivation, storage at 4$\\sp\\circ$C, and to purification when compared to the ascorbate-supplemented animals. Analysis of the composition of the microsomal membrane showed no differences in the total phospholipid, phosphatidylcholine, or cholesterol content between the ascorbate-supplemented and ascorbate-deficient animals. However, there was a significant increase in membrane fluidity in ascorbate-deficiency which was not reversed with the in vitro addition of ascorbic acid. A quantitative decrease in ascorbate-deficient guinea pig UDPGT compared to the ascorbate-supplemented was suggested by reduced protein b and ing at 55,000 daltons. Underst and ing how dietary ascorbic acid influences UDP-glucuronyltransferase is important since alterations in its activity may result in increased toxicity of environmental chemicals, pharmaceutical agents, and endogenous compounds.