Histidyl Residues Mediate Electron Transport in Plant Mitochondria*

The effect of chemical modification of histidyl residues using diethyl pyrocarbonate (DEP) on plant mitochondrial electron transport was studied. Mitochondrial membranes from potato tubers were isolated and electron flow from NADH to oxygen, NADH to ferricyanide and ascorbate to oxygen were monitored in pro», nee and/or absence of DEP. Measurements were made at various concentrations of DEP and at different pHs either by using an oxygen electrode or spectrophotometrically. The results show that DEP inhibits flow of electrons from NADH to oxygen, however partial electron transport from NADH to ferricyanide and ascorbate to oxygen was unaffected. Maximum inhibition was observed at pH 6.5. The time course of the DEP action revealed a biphasic nature of inhibition. Effects on the levels of reduction of cytochromes b and c by DEP during electron transport indicated that histidyl residues may be present before or at cytochrome b , which are being modified.