Profiles of creatine kinase isoenzyme compositions in single muscle fibres of different types.

Creatine kinase (CK) isoenzyme compositions of different types of single muscle fibres dissected from soleus (SOL) and extensor digitorum longus (EDL) muscles from rats were examined. CK isoenzymes were separated into cytoplasmic (CK-MM, CK-MB, CK-BB) (muscle, brain and hybrid types, respectively) and mitochondrial (m-CK) isoenzymes. Total CK and CK-MM activities showed the highest activities in fast-twitch glycolytic fibres (FG), lower in fast-twitch oxidative glycolytic (FOG) and the lowest in slow-twitch oxidative (SO) fibres. Conversely, the activity of m-CK was highest in SO, lowest in FG and intermediate in FOG fibres. The activity of CK-MB was highest in SO and lower in FOG and FG fibres. However, the activities of total CK and CK isoenzymes in a single muscle fibre type were not distinguishable from those of another type, and the profiles of CK isoenzyme compositions from the same type of single muscle fibres overlapped over a considerable range. The relationships between the four CK isoenzymes activities in single muscle fibres of different types were not similar. These results suggest that CK isoenzymes of single muscle fibres of different types play different roles in intracellular energy metabolism. Therefore, it is supposed that the CK isoenzyme compositions of single muscle fibres are suitable for their contractive and metabolic properties.