Practolol metabolism. III. Irreversible binding of [14C]practolol metabolite(s) to mammalian liver microsomes.

The irreversible binding of [14C]practolol metabolites to mammalian liver microsomes was studied in vitro. Binding occurred by covalent linkage to amino acids of microsomal proteins. The binding reaction was dependent on oxygen, NADPH, reaction time, microsomal protein content and substrate concentration and was inhibited by carbon monoxide, glutathione and albumin. Albumin inhibition resulted in the binding of reactive metabolite(s) to this serum protein. Hamster (Syrian) liver microsomes had the highest rate of binding and 3-hydroxypractolol formation and marmoset the lowest. The reduced binding of [acetyl-14C]practolol compared to [phenyl-14C]practolol to hamster liver microsomes, but not rat, indicated that metabolites with and without the acetyl grouping were bound in this species. The binding of [14C]desacetylpractolol to hamster liver microsomes was greater than the binding of practolol. Sodium fluoride caused both an increase and a decrease of practolol binding to microsomes from pooled hamster livers, whereas there was a consistent increases in binding to microsomes from individual hamster livers. The collective data indicate that electrophilic metabolites of practolol, possibly N-hydroxylation of the acetyl-amino grouping are formed by the microsomal cytochrome P-450-dependent drug metabolizing enzyme system. These metabolites can react irreversibly with tissue and serum proteins which could be sufficiently altered to be recognized as antigens.