Effects of temperature and substrate concentration on the LDH/α-HBD ratio in heart and liver disease

The use of an optimal 2-oxobutyrate concentration at 37° results in the loss of increased affinity of this substrate for the electrophoretically fast moving cardiac isoenzymes. Optimized pyruvate and 2-oxobutyrate concentrations result in the loss of the diagnostically useful LDH/α-HBD ratio in cases of cardiac and hepatic disease. There is no combination of optimal and sub-optimal concentrations at 37° which restores this ratio. The ratio loss is therfore due to the increased temperature as well as the increased substrate concentrations. However, the diagnostic value of the enzymes is not diminished when they are measured at 37°using optimal conditions. The loss of the ratio does not cause diagnostic problems and the optimized methods at 37° have the advantage of enabling a larger number of enzyme measurements to be carried out.