Abstract
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 musomes 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 K m of 5.8 μM was obtained from linear Lineweaver-Burk plots together with a V max of about 150 pmoles/mg musomal protein/min. (3) Neither phenobarbital nor polycyclic aromatic hydrocarbons caused a considerable induction of the enzyme without change of the apparent K m , but spironolactone did. (4) The conjugation of the substrate (4 μM) could only be inhibited by the 3'-epi-digitoxoside of digitoxigenin. (5) 25–50 μM 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.
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