Enzymatic assessment of myocardial injury after infarction or cardiac surgery. Is isoenzyme analysis required?

Estimation of enzyme release in plasma requires knowledge of the fractional catabolic rate constant (FCR) for the elimination enzyme activity from plasma. However, the total plasma content of such enzymes usually consists of several isoenzymes with different values of FCR. Thus, the use of a single overall value for FCR may cause error. This problem was studied by determination of the plasma isoenzyme activities of creatine kinase, lactate dehydrogenase, aspartate aminotransferase and alpha-hydroxybutyrate dehydrogenase in patients after cardiac surgery and after acute myocardial infarction. Values of FCR and the cumulative release of activity in plasma are estimated for separate isoenzymes and for total enzyme activity. Results are compared with the enzyme content of myocardium, skeletal muscle and blood cells. It is concluded that isoenzyme separation is not required for the quantitative use of such data. The implications for the validation of enzymatic estimation of cardiac injury are discussed. The results indicate that local inactivation of enzymes after cardiac injury must be limited.