Reduction of tertiary amine N-oxides by rat liver mitochondria

Reduction of tertiary amine N-oxides by the mitochondrial fraction of rat liver was investigated. NADPH was required as a cofactor. The rate of reaction was faster with the NADPH-generating system than with NADPH. Isocitrate in the NADPH-generating system revealed the maximum stimulation. A little less activity was observed when NADH was used as a cofactor instead of NADPH. The reductase activity was markedly inhibited under aerobic conditions. The rates of mitochondrial tertiary amine N-oxide reduction expressed in nanomoles per milligram of protein per minute were: N,N-dimethylaniline N-oxide, 4.3; tiaramide N-oxide, 0.47; and imipramine N-oxide, 0.14. The activity for N,N-dimethylaniline N-oxide was comparable to the microsomal activity, but the activity for imipramine N-oxide was much less than that in microsomes. Isocitrate and alpha-ketoglutarate were found to stimulate mitochondrial tertiary amine N-oxide reduction more efficiently than other Krebs cycle intermediates. Oxalacetate, on the other hand, was the least effective intermediate. ATP together with NADPH and NADP stimulated the reaction efficiently, probably due to the energy-dependent intramitochondrial transhydrogenation. Antimycin, rotenone and cyanide had little or no effect on isocitrate-dependent N-oxide reduction, whereas an inhibitory effect was observed on succinate-supported N-oxide reductase. N-oxide reductase activity in mitochondria was partially suppressed under an atmosphere of carbon monoxide, althouth no increase of the activity was observed by phenobarbital pretreatment, nor inhibition by 2,4-dichloro-6-phenylphenoxyethylamine. Experiments with digitonin-treated mitochondria demonstrated that mitochondrial N-oxide reductase was bound to mitochondrial inner membrane or its matrix.