SOME CHARACTERISTICS OF HEPATIC MICROSOMAL SYSTEMS WHICH METABOLIZE AMINOPYRINE IN THE RAT AND RABBIT

¹ Department of Pharmacology, College of Medicine, University of Iowa, Iowa City, Iowa

The metabolism of aminopyrine, as measured by the formation of 4-aminoantipyrine (4-AAP) and of formaldehyde, was studied in hepatic microsomal preparations from control and phenobarbital-treated rats and rabbits. Substrate optima, V_max and K_m for aminopyrine metabolism as measured by production of either 4-AAP or formaldehyde were lower in rat-liver preparations than in rabbit liver. Both 4-AAP and formaldehyde were formed from aminopyrine by hepatic microsomal systems which required , reduced nicotinamide adenine dinucleotide phosphate and oxygen. Phenobarbital pretreatment increased the formation of 4-AAP relatively more than that of formaldehyde in the rat. The ratio of formaldehyde formed to 4-AAP produced was invariably greater than 2:1 in both rats and rabbits. The quantity of formaldehyde or 4-AAP formed per micromole of aminopyrine added and the ratio of formaldehyde produced per unit of 4-AAP formed by 9000 x g supernatant fractions from liver of the rat or rabbit suggested that formaldehyde and 4-AAP production from aminopyrine varied with the incubation time and the concentration of added aminopyrine. The results could be explained if the removal of one methyl group from aminopyrine to form monomethyl-4-aminoantipyrine (MMAP) were a fast reaction and if the further dealkylation MMAP to 4-AAP were slow. Thin-layer chromatography showed that incubation of aminopyrine with fortified hepatic 9000 x g supernatant fractions led to the formation of relatively large amounts of MMAP but only small quantities of 4-AAP. Excess aminopyrine was found to inhibit the demethylation of MMAP to 4-AAP, both in rat and rabbit liver.