Prizidilol: Metabolism by cytochrome P-450 and acetyltransferase

Abstract

The hepatic microsomal cytochrome P-450 enzyme system bound and metabolized the experimental drug prizidilol. Prizidilol bound to two distinct sites on cytochrome P-450. At low concentrations (less than ca 20 μM), prizidilol bound to the substrate binding site of the enzyme and produced a Type I difference spectrum. At higher concentrations (25–190 μM), prizidilol bound to the oxygen binding site of the enzyme and produced a type II difference spectrum. Prizidilol stimulated hepatic microsomal CO-inhibitable NADPH oxidation. Prizidilol metabolism by hepatic microsomes assessed by prizidilol disappearance was inhibited by CO:O 2 (80:20; v/v), SKF 525-A and metyrapone. Prizidilol disappearance was monitored using a newly developed TLC assay for prizidilol following derivatization with quinolin-3-al. The apparent binding constants ( K s), maximum extents of binding ( ΔA max), Michaelis constants ( K m) and maximum velocities ( V max) for the interaction of prizidilol with hepatic microsomal cytochrome P-450 were assessed in rats pretreated or not with the inducing agents phenobarbital, β-naphthoflavone and pregnenolone-16α-carbonitrile. For the differently pretreated rats the apparent K s values for the type I site and the type II site and the apparent K m were ca 3 μM, 150 μM and 2 μM, respectively. Apparent V max values varied from 20 to 70 pmol per min per mg microsomal protein. The observed effects of induction on the apparent equilibrium constants and maximum extents of binding and metabolism of prizidilol indicate that the forms of cytochrome P-450 induced by phenobarbital, pregnenolone-16α-carbonitrile or β-naphthoflavone do not play a major role in the metabolism of prizidilol. Prizidilol was also metabolized by hepatic cytosolic N-acetyltransferase. The apparent K m values for prizidilol and acetyl CoA were 0.8 and 22 μM. Apparent V max values were 50 and ca 2 pmol per min per mg protein for partially purified transferase and cytosol, respectively. It is concluded that the rates of oxidation and acetylation of this drug would be expected to be relatively low, being limited by low apparent V max values for both oxidation and acetylation.