Studies on adenosine triphosphate transphosphorylases. XVII. A physicochemical comparison of the ATP-creatine transphosphorylase (creatine kinase) isozymes from man,calf, and rabbit

All of the creatine kinase isozymes from human, calf, and rabbit brain and muscle are composed of two noncovalently linked polypeptide chains, based upon sedimentation equilibrium analyses in the presence and absence of disruptive agents. The brain-type isozymes of man, calf, and rabbit proved to be slightly heavier than the muscle types. Various physicochemical properties of the isozymes are recorded. Each group of isozymes, i.e., the muscle, hybrid (muscle-brain), and brain isozymes from man, calf, and rabbit, showed similar electrophoretic behavior, although isoelectric points were not precisely identical for the muscle and hybrid types. Theoretical titration curves constructed from amino acid compositions of the calf isozymes showed reasonable agreement between their calculated and measuredpI ₀ values (isoelectric point extrapolated to zero ionic strength). The three native muscle isozymes and brain isozymes all contain two reactive sulfhydryl groups per mole or one per polypeptide chain of their two-chain proteins, which may be titrated with 5,5′-dithiobis (2-nitrobenzoic acid); and under acidic conditions, quantitative titrations with 4,4′-dithiodipyridine yield a total of ten- SH groups per mole of each brain-type and eight- SH groups per mole of muscle-type isozyme in the case of man, calf, and rabbit. A comparison of their amino acid compositions and tryptic peptide maps shows that there is only a slightly greater degree of homology between the individual isozymes of the same type (muscle type or brain type) than between the muscle- and brain-type isozymes of the same species.