Labeling of the electron entry site of cytochrome c oxidase using 51Cr2+

51Cr2+ has been used as a probe to locate the electron entry site of bovine cytochrome c oxidase. The results of static titrations, column chromatography, and low pH LDS polyacrylamide gradient gel electrophoresis are reported. Of the protein subunits of cytochrome c oxidase, only subunit II is specifically labeled during electron transfer from Cr2+ to the electron accepting site. We therefore conclude that this site is located in subunit II. Our results provide experimental evidence to corroborate the view that this subunit is associated with redox centers of the enzyme, an hypothesis based on indirect evidence provided by the amino acid sequences and analogy with the bacterial enzyme.