Some Interrelations between Carboxymethylation and Heme Reactions in Sperm Whale Myoglobin

The effect of carboxymethylation of sperm whale ferrimyoglobin on the conversion of the protein to the deoxymyoglobin (ferromyoglobin), oxymyoglobin, and carboxymyoglobin forms was studied. Judged by observations on the absorption spectra and on optical rotation at 233 mµ, the conversions to the other heme states were largely unaffected by carboxymethylation to the extent of an average of 7 out of the 12 histidine residues; this left an average of 5 histidine residues unchanged. If the carboxymethylation reaction was carried farther, however, clear evidence of altered properties was obtained. Some evidence was obtained which implied that formation of the carboxymyoglobin derivative could drive the somewhat altered structure back to a form close to the native structure. The course of carboxymethylation of ferromyoglobin and carboxymyoglobin was also followed. In both cases the reaction followed much the same course as that with ferrimyoglobin. Under comparable conditions slightly more histidine residues proved to be modified in the reaction with carboxymyoglobin than with ferrimyoglobin, an observation reflected in a higher content of 1-carboxymethylhistidine. It is suggested that the difference in content of 1-carboxymethylhistidine may imply a greater reactivity of the E7 histidme residue. With the exception of the possibility raised above of a slightly higher reactivity at the nitrogen 1 of a histidine residue in the carboxymyoglobin form, the results indicate very similar reactivity patterns of exposed histidine residues in ferrimyoglobin and in ferromyoglobin.