An isoenzyme of creatine kinase [ATP: creatine N-phosphotransferase, EC 2.7.3.2] was isolated in pure, crystalline form from mitochondria of human heart muscle. The enzyme has a molecular weight of 84,000 and consists of two subunits with identical molecular weight, each containing two reactive sulfhydryl groups. The enzyme has a specific activity of 15 +/- 2 U/mg in the direction of creatine phosphate synthesis at optimum pH of 8.7 and of 45 +/- 5 U/mg in the direction of ATP synthesis at optimum pH of 6.7. Mitochondrial creatine kinase has Michaelis constants of 1.70 mM for MgATP2-, 8.00 mM for creatine, 0.15 mM for MgADP-, and 3.00 mM for creatine phosphate. The mitochondrial enzyme differs from the other creatine kinase isoenzymes, i.e. from muscle (CK-MM), brain (CK-BB), and their hybrid form (CK-MB) in (I) its amino acid composition and its amino terminal amino acid sequence, (II) its electrophoretic mobility, and (III) its immunological properties. It thus constitutes a fourth isoenzyme of creatine kinase. By analogy to CK-MM, CK-BB, and CK-MB, the mitochondrial isoenzyme was designated as CK-MiMi.
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Blum et al. (1983) studied this question.