Activities of key enzymes in the energy metabolism of human myocardial and skeletal muscle.

Activities of total creatine kinase (CK), its isoenzyme MB (CK-MB), total lactate dehydrogenase (LD) and its isoenzyme LD1, phosphofructokinase (PFK), aspartate aminotransferase (ASAT) and citrate synthase (CS) were determined in skeletal muscle biopsies obtained from physically trained and untrained men and in myocardial biopsies from patients subjected to open heart surgery because of valve disease. The LD1, ASAT and CS activities were higher in trained than in untrained skeletal muscle and still higher in heart muscle than in either trained or untrained skeletal muscle. The CK-MB activity was higher in trained than untrained skeletal muscle and the myocardial CK-MB activity was similar to that in trained skeletal muscle. Total CK activity was slightly lower in trained than in untrained skeletal muscle and the myocardial CK activity was approximately one third of the skeletal muscle CK. Both the PFK and the total LD activity was of similar magnitude in the different muscle types. In conclusion, as estimated by enzyme activities, the oxidative capacity is 2-3 times larger in myocardial than in skeletal muscle, while the glycolytic capacity as estimated by PFK appears to be the same.