Apparent Michaelis constants for the metabolism of [ureyl- 14C]tolbutamide by human liver microsomal preparations

Abstract

A method is described for the measurement of the amount of l-butyl-3- p-hydroxymethylphenylsulphonyl [ 14C]-urea (hydroxy-[ureyl- 14C]tolbutamide) formed in vitro from [ureyl- 14C]tolbutamide. Very low rates of metabolism can be assayed accurately. K m and V max have been determined for the metabolism of [ureyl- 14C]-tolbutamide by microsomal preparations from human liver obtained (a) postmortem; (b) as operation biopsy material; (c) from tissue frozen for 1 week at −20°. Frozen microsomal pellets have also been investigated. No significant differences have been found between the Michaelis constants for the four types of preparation, although the values reported are generally lower than those in the literature for the metabolism of other drugs by human liver preparations. Pentobarbitone inhibits the rate of metabolism in vitro of [ureyl- 14C]tolbutamide in a manner neither fully competitive nor fully non-competitive.