Amino acid composition of horse heart cytochrome c.

As the first step in the study of the amino acid sequence of horse heart cytochrome c, it was essential to establish the exact composition of the protein. Although the molecular weight is low (approximately 12,000) (1) and the protein contains less than 110 amino acid residues per mole, analyses by three diierent laboratories (2-4) have not yielded strictly concordant results. These analyses are in agreement to within 1 residue per mole for the content of arginine, proline, methionine, isoleucine, tyrosine, and phenylalanine but vary by 1 residue or more for all of the other amino acids. The relatively recent development of highly accurate automatic recording equipment for the analysis of amino acid mixtures (5) and of an enzymic method for the complete hydrolysis of proteins (6) that obviates some of the difficulties encountered with acid hydrolysis procedures has made it possible to determine precisely the composition and the amide distribution of horse heart cytochrome c.