Abstract
Several cysteine residues are alkylated per flavin during the irreversible inactivation of purified succinate dehydrogenase (8 g atoms of iron per mole of flavin) by N-ethylmaleimide. Experiments involving malonate, which partially protects the enzyme from inactivation, distinguish between alkylation events which are crucial to enzymatic activity and those which are not crucial. There appears to be one crucial cysteine per flavin and this cysteine is located on the flavoprotein subunit. Both N-ethylmaleimide and bromopyruvate alkylate a similar set of residues on succinate dehydrogenase and show similar kinetics in the destruction of enzymatic activity.