Brain isoenzyme of creatine kinase. II. Species specificity of enzyme and presence of inactive form in striated muscle of rabbit and man.

Antibodies specific for the brain isoenzymes (BB) of creatine kinase were prepared by the injection of the highly purified rabbit enzyme into roosters. The antibodies were found to cross-react only partially with BB of rat, guinea pig, sheep, monkey, and man, demonstrating that these proteins are not entirely identical with respect to their antigenic sites. A greater degree of species specificity seems to exist for BB than for the muscle isoenzyme (MM) suggesting that BB may have undergone considerable evolutionary modification. Developmentally mature rabbit cardiac and skeletal muscles contained large quantites of an enzymatically inactive BB which can be detected immunochemically. It is antigenically identical with active rabbit BB but the two proteins can be separated by DEAE-cellulose chromatography. Enzymatically inactive BB is also present in human skeletal muscle. The presence of MM in muscle provides sufficient catalytic function and may have permitted the evolution of inactive BB in an additional (noncatalytic) metabolic role.