New instrument for rapid determination of activities of lactate dehydrogenase isoenzymes.

Lactate dehydrogenase isoenzymes can be distinguished kinetically by the fact that isoenzyme H is strongly inhibited a few seconds after the reaction is started if high concentrations of pyruvate are present, in contrast to the M isoenzyme. A new instrument that exploits this fact can measure both the total activity and the proportion of H isoenzyme in serum or plasma in 8 to 10 s. The instrument consists of a simplified stopped-flow apparatus in which the plasma is assayed for lactate dehydrogenase activity, and an electronic device that measures the rate of the reaction at two pre-set time intervals. The first rate is taken between 0.2 and 0.4 s after the reaction is started, a time at which both isoenzymes are fully active, and at which the rate obtained thus reflects total lactate dehydrogenase activity in the plasma sample. The second rate is measured 4 to 6 s after the start of the reaction, at which time the H isoenzyme has become inhibited and the observed rate compared to the initial rate is therefore proportional to the percentage of H isoenzyme activity in the serum. These two rates are electronically displayed on two three-digit voltmeters, the first display being the total activity, the second a number proportional to the inhibited slope. The percentage of M isoenzyme can then be calculated from the initial and final rate. A total of five to six repeat assays may be done within a minute on 1 ml of plasma or serum. This instrument may be of significant value in following the progress of myocardial infarctions and other diseases.