Stability in Urea as a Measure of Cardiac Lactate Dehydrogenase on the CentrifugalAnalyzer

Kinetic methods are described for determination of total, heat-stable, and urea-stable lactate dehydroge-nase (EC 1.1.1.27) activity on the centrifugal analyz-er. In the urea-stable method we present, 2.6 molar urea is used to differentiate lactate dehydrogenase originatingfrom heart and liver.This urea concentra-tion, greater than that used by most other investiga-tors, better differentiates between cardiac and liver lactate dehydrogenase and is as sensitive and spe-cific for cardiac lactate dehydrogenase as the heat-stable method; excellentcorrelationwas obtained on 200 specimens assayed by both methods. The urea-stable method has the advantages of speed and sim-plicity, better precision, and decreased serum vol-ume, and is, therefore, preferred to the heat-stable method for determination of cardiac lactate dehydro-genase isoenzymes. Additional Keyphrases: kinetic enzyme assay #{149}diag-nostic aid #{149} normal values #{149}inter-method compari-son Measurement of serum total LD1 activity is a rela-tively nonspecific index of tissue damage; therefore, methods that measure the LD isoenzymes have been developed. Measurement of cardiac LD1, the most anodal isoenzyme on electrophoresis at pH 8.6, is important in the diagnosis of myocardial infarction, especiallywhen injury to organs other than the heart isknown or suspected. A variety of methods based on the differentphysi-cal and chemical properties of LD1 and of LD5, the most cathodic isoenzyme found primarily in liver and skeletal muscle, have been developed, but those most commonly used are the HBD assay (1, 2), the heat-stable assay (3-6), and urea-stable assay (7-11). In the lastseveralyears the diagnostic specifici-ty of the HBD assay when run at temperatures othe