Microsomal electron transport reactions: III. Cooperative interactions between reduced diphosphopyridine nucleotide and reduced triphosphopyridine nucleotide linked reactions

Abstract

DPNH, when added in the presence of a TPNH-generating system with steadystate concentrations of TPNH as low as 1 μ m, significantly affects the rate of formaldehyde formation during the rabbit liver microsome catalyzed oxidative demethylation of aminopyrine. Further, low steady-state concentrations of TPNH can cause a substantial increase (up to fourfold) in the rate of DPNH oxidation by microsomes. The unexpected ability of DPNH to markedly stimulate the rate of aminopyrine metabolism in the presence of either TPNH or TPN + plus a TPNH-generating system is not explained by any of the existing schemes of microsomal electron transport. The same is true for the ability of low levels of TPNH to significantly increase the rate of DPNH oxidation. The present paper suggests two possible mechanisms for microsomal electron transport to account for the cooperative interaction between TPNH and DPNH. These findings are considered of physiological significance since the normal cytosol environment of the hepatic cell contains both TPNH and DPNH.