A comparison of the kinetic properties of two different forms of microsomal UDPglucuronyltransferase.

Two forms of UDPglucuronyltransferase (EC 2.4.1.17) have been purified from microsomes of pig liver. One form is free of phospholipids and the other contains a small amount of residual phospholipids. Each form, however, is responsive to activation on addition of purified phospholipids. Comparison of kinetic properties of these enzymes, after reconstitution with identical phospholipid environments, indicate that these are unique functional forms of UDPglucuronyltransferase. The two differ by as much as 100-fold in their rates of conjugation at Vm of p-nitrophenol. Relative rates of glucuronidation of a variety of phenolic aglycones are different for the two enzymes, which suggests different reaction mechanisms. The energetic basis for binding of UDP-glucuronic acid to the active sites is different for the two forms of UDPglucuronyltransferase. Moreover, one form, but not the other, binds Mn2+, which leads to modulation of kinetic properties.