STUDIES ON THE REDUCTION OF AROMATIC NITRO GROUPS IN HUMAN AND RODENT PLACENTAL HOMOGENATES

¹ Department of Biochemical Pharmacology, State University of New York, Buffalo, New York

The capacities of human, rat and mouse placental homogenates to catalyze reduction of the aromatic nitro group of p-nitrobenzoic acid to the corresponding amine were investigated. The reduction reaction was readily demonstrated in 9000 and 104,000 x g supernatant fractions of rat and mouse placental homogenates but could be observed in human placental homogenates only upon addition of flavins to incubation mixtures. Activity in rat and mouse placental homogenates was abolished by dialysis but was restored by addition of flavins to incubation mixtures. Ribofiavin-5-phosphate was particularly effective in restoring activity. Pretreatment of experimental animals with 3-methylcholanthrene or phenobarbital at various stages enhanced the reduction reaction in maternal liver homogenates but not in placental homogenates of the same animals. The reaction was found to proceed via both enzymic and nonenzymic mechanisms involving reduced nicotinamide ademne dinucleotide phosphate or reduced nicotinamide adenine dinucleotide generating systems, nonenzymic reduction of flavins by either agent, and nonenzymic reduction of the aromatic nitro group by a reduced flavin. Anaerobic incubation conditions were required to prevent rapid reoxidation of reduced flavins and/or reduced intermediates, and the reaction was markedly inhibited by both oxygen and carbon monoxide. Homogenate was required for the nonenzymic reduction of the flavins and a protein-bound, heat-stable intermediate appeared to be essential for the reaction. The intermediate did not appear to be localized in any particular placental homogenate subfraction.