Possible occurrence and role of an essential histidyl residue in succinate dehydrogenase

Diethylpyrocarbonate (Et2PC) inhibits the succinate dehydrogenase [succinate:(acceptor) oxidoreductase, EC 1.3.99.1] activity of submitochondrial particles, Complex II (succinate:ubiquinone oxidoreductase), and the soluble, pure succinate dehydrogenase. The reaction order with respect to Et2PC concentration is close to unity, suggesting modification of one essential residue per active unit of the enzyme. The pH profile of Et2PC inhibition, the partial reversal of inhibition by hydroxylamine, and the spectral change of the Et2PC-treated enzyme in the UV region suggest modification of a histidyl residue. Succinate dehydrogenase activity can be protected against Et2PC inhibition by succinate, fumarate, malonate, or oxaloacetate (also by activating anions such as ClO4(-) and Br-), suggesting that the Et2PC-modified essential residue might be at the active site. In both submitochondrial particles and the purified enzyme, succinate dehydrogenase activity is highest and relatively constant at pH greater than or equal to 7.0 and diminishes precipitously at pH less than 7.0. By contrast, fumarate reductase activity is highest at pH less than or equal to 7.0 and diminishes at pH greater than 7.0. These results are consistent with the possible participation of the unprotonated and protonated forms of the imidazole moiety of the putative histidyl residue, respectively, in succinate oxidation and fumarate reduction.