Substrate specificity and characterization of rat liver p-nitrophenol, 3 alpha-hydroxysteroid and 17 beta-hydroxysteroid UDP-glucuronosyltransferases.

Purified preparations of rat liver 17-hydroxysteroid, 3-hydroxyandrogen and p-nitrophenol (3-methylcholanthrene-inducible) UDP-glucuronosyltransferases were further characterized as to their substrate specificities, phospholipid-dependency and physical properties. The two steroid UDP-glucuronosyltransferases were shown to exhibit strict stereospecificity with respect to the conjugation of steroids and bile acids. These enzymes have been renamed 17 beta-hydroxysteroid and 3 alpha-hydroxysteroid UDP-glucuronosyltransferase to reflect this specificity for important endogenous substrates. An endogenous substrate has not yet been identified for the p-nitrophenol (3-methylcholanthrene-inducible) UDP-glucuronosyltransferase. The steroid UDP-glucuronosyltransferase activities were dependent on phospholipid for maximal catalytic activity. Complete delipidation rendered the UDP-glucuronosyltransferases inactive, and enzymic activity was not restored when phospholipid was added to the reaction mixture. After partial delipidation, phosphatidylcholine was the most efficient phospholipid for restoration of enzymic activity. Partial delipidation also altered the kinetic parameters of the 3 alpha-hydroxysteroid UDP-glucuronosyltransferase. The three purified UDP-glucuronosyltransferases are separate and distinct proteins, with different amino acid compositions and peptide maps generated by limited proteolysis with Staphylococcus aureus V8 proteinase. Some similarity was observed between the amino acid composition and limited proteolytic maps of the steroid UDP-glucuronosyltransferases, suggesting they are more closely related to each other than to the p-nitrophenol UDP-glucuronosyltransferase.