The molecular basis of the heterogeneity of the MM isozyme of rabbit muscle creatine phosphokinase

Identification by isoelectric focusing of creatine kinase-MM isoforms in the plasma and striated muscle of pigs

Total creatine kinase (CK) and CK-MM isoforms were determined in plasma and longissimus dorsi muscle extracts from normal pigs. Based on their total CK activity, the pigs were divided into two groups. Pigs of group 1 (n = 16) had a mean plasma total CK of 298 +/- 16 U/L and the distribution of the CK-MM isoforms was 65.7 +/- 2.5% CK-MM3, 18.9 +/- 1.6% CK-MM2 and 15.3 +/- 1.5% CK-MM. In group 2 (n = 18; 826 +/- 75 U/L total CK) four isoforms were observed: 3.1 +/- 0.9% CK-MM, 67.9 +/- 3.0% CK-MM3, 21.5 +/- 2.3% CK-MM2 and 7.5 +/- 1.3% CK-MM. The differences between the two groups of pigs were significant (p less than 0.001) for CK-MM1 and the presence of CK-MM. Four CK-MM isoforms were also detected in longissimus dorsi muscle homogenates: 45.6 +/- 8.1% CK-MM, 32.6 +/- 11.7% CK-MM3, 16.6 +/- 2.3% CK-MM2 and 5.1 +/- 2.8% CK-MM1. The release of CK-MM isoforms from muscle into plasma seems to be unrelated to the concentration of these isoforms in striated muscle.

N-terminal sequence of creatine kinase from skeletal muscle of rabbit and rhesus monkey

The first 20 amino acids from the N-terminus of skeletal muscle (MM) creatine kinase from both rabbit and rhesus monkey have been identified and these sequences show considerable homology. Contrary to an earlier report, the N-terminus was not found to be blocked. Both of these sequences show much less homology with the N-terminal sequence of heart muscle (MM) creatine kinase and no homology with that of the heart muscle mitochondrial (MiMi) isozyme. No homology was found between the N-terminal sequence of the mitochondrial isozyme and the URF (unidentified reading frame) proteins of the human mitochondrial genome, indicating that the mitochondrial enzyme is encoded by nuclear genes. This suggests the possibility that an N-terminal peptide may be cleaved from the mitochondrial isozyme on its translocation across the mitochondrial membrane.

Investigation of the heterogeneity of the MM isozyme of creatine kinase

MM creatine kinase (MM CK) from human skeletal muscle was resolved into 21 subspecies by isoelectric focusing. Rabbit and bovine MM CK's were also shown to have patterns of 21 bands. These results are compatible with the presence of six different M subunit forms in skeletal muscle, which by random association into dimers, could produce 21 different subspecies of creatine kinase. Two-dimensional isoelectric focusing of human MM CK yielded a diagonal pattern, indicating that the various enzyme subspecies were not induced by ampholyte-protein interaction. Heat inactivated serum appeared to stabilize various CK subspecies and also produced one further anodally migrating MM CK form. Fresh serum had combined inhibitory and stabilizing effects on the MM CK subforms, and allowed for the identification of four anodally migrating CK subspecies not present in tissue extract. These findings are of critical importance because of the widespread use of CK isoenzymes in serum for diagnostic purposes.

Occurrence of heterogenous forms of the subunits of creatine kinase in various muscle and nonmuscle tissues and their behaviour during myogenesis

Purified, homodimeric creatine kinases from chicken were subjected to two-dimensional gel analysis under dissociating conditions. Each of the subunits M-creatine kinase and B-creatine kinase was resolved into a basic and an acidic subspecies with very similar mobilities in the sodium dodecylsulfate dimension. The M-creatine kinase subspecies were found in myogenic cells, fast muscle, slow muscle and the B-creatine kinase subspecies were present in heart, gizzard and brain. The creatine kinase subunits were identified in these tissues by a variety of methods like immunoreplicas of two-dimensional gels, immunoprecipitations, or coelectrophoresis with purified creatine kinase and all gave the same results. In the course of myogenic development in vitro the subspecies were synthesized coordinately and no indication was found for a differential regulation of any of the subspecies of the creatine kinase subunits. No radioactive phosphorus was incorporated into either one of the subspecies, hence phosphorylation could be ruled out as the source of heterogeneity. Furthermore, peptide mapping analysis of partial proteolytic digests did not reveal differences among the subspecies of the same subunit. Not only chicken but also rat creatine kinase displayed this type of heterogeneity. All subspecies were observed after translation of chicken RNA in a cell-free protein-synthesizing system. The heterogeneity probably might best be explained by the existence of multiple, but closely related genes for the creatine kinase subunits.

A study on the dimeric structure of creatine kinase (EC 2.7.3.2)

The generally accepted dimeric structure of creatine kinase (CK) is one in which M subunits and B subunits can occur. This paper reports the existence of three MM-bands and two MB-bands in human sera with increased CK-activity. These findings and supplementary data gathered from hybridisation experiments have led to the conclusion that there are two different M subunits, which can both occur in the enzyme in vivo. Further, findings are reported on the different CK-isozyme patterns of extracts from the cerebellum and from the cortex and the medulla of the cerebrum.