On the substrate specificity of the digitoxigenin monodigitoxoside conjugating UDP-glucuronyltransferase in rat liver.

The aim of the present study was to investigate the specificity of the UDP-glucuronosyltransferase (EC 2.4.1.17) involved in the conjugation of digitoxigenin monodigitoxoside. By in vitro assays with detergent activated liver microsomes it was found that (1) digitoxigenin monodigitoxoside is by far the best substrate of all cardenolides and cardenolide digitoxosides tested. (2) In the presence of saturating UDP-glucuronate concentrations an apparent Km of 5.8 microM was obtained from linear Lineweaver-Burk plots together with a Vmax of about 150 pmoles/mg microsomal protein/min (3) Neither phenobarbital nor polycyclic aromatic hydrocarbons caused a considerable induction of the enzyme without change of the apparent Km, but spironolactone did. (4) The conjugation of the substrate (4 microM) could only be inhibited by the 3'-epi-digitoxoside of digitoxigenin. (5) 25-50 microM substrate inhibited only the conjugation of the 3'-epimer and that of digoxigenin monodigitoxoside. It is suggested that there is a form of glucuronyltransferase which specifically conjugates digitoxigenin monodigitoxoside.