The reactivity to N-ethyl maleimide of the subunits of cytochrome oxidase

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Abstract
Beef heart cytochrome oxidase (EC 1.9.3.1) consistently presented 10 Coomassie blue staining zones on SDS-polyacrylamide gel electrophoresis. At pH 7.0 only 2 of these polypeptides (III and VIa) are labeled by radioactive N-ethyl maleimide (NEM). The labeling of VIa was variable and correlated with the activity of particular oxidase preparations. When cytochrome oxidase was isolated from alkylated membranes, either mitochondria or electron transport particles, polypeptide VIa was not labeled; polypeptide III was more strongly labeled than when isolated oxidase was alkylated, and label now appeared in polypeptide I which was not alkylated upon treatment of isolated oxidase with NEM.