Further studies on the structure and properties of human haemoglobins modified by digestion with the carboxypeptidases

The two derivatives of human adult haemoglobin, Hb-CPA and Hb-CPB, which result from the separate action of carboxypeptidases A and B on Hb-A, give all the soluble tryptic peptides normally derived from Hb-A except for the C-terminal peptides βTpXV and αTpXIV of the β^A- and α^A-chains respectively. The appearance in the peptide pattern of Hb-CPA of the penultimate tryptic peptide, βTpXIV, at the C-terminus of the β^A-chain, confirms that only the terminal tyrosine and histidine residues have been removed. The corresponding peptide from the α^A-chains is insoluble but the presence of free tyrosine in the tryptic digest of Hb-CPB is consistent with the loss of only the C-terminal arginine residue from the α^A-chain. The subunits of both modified haemoglobins combine with the subunits of other haemoglobins to form hybrid haemoglobin species. Attempts to isolate a haemoglobin derivative in which only one of the two α^A-chains had been digested with CPB were unsuccessful, and the reasons for this are discussed.