Abstract
A peptide containing an essential tyrosine residue, modified with the nitrobenzofurazan group, was purified from bovine heart mitochondrial ATPase. The composition of the peptide indicates that this tyrosine is residue 311 in the .beta. chain sequence. The problem of the instability of the tyrosyl-nitrobenzofurazan bond was overcome by working throughout at relatively acidic pH and using pepsin digestin of the enzyme.

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