True Arrhenius relationships of human lactate dehydrogenase

The pyruvate and NADH concentrations and the buffer pH which gave maximal activity with LDH isoenzymes derived from human heart and liver tissue were established for the temperatures 25 degrees C, 30 degrees C, 35 degrees C, 37 degrees C,, 40 degrees C, 45 degrees C, and 50 degrees C. The velocities of the LDH isoenzymes using these maximal assay conditions were used to obtain Arrhenius plots, i.e. log initial velocity against inverse absolute temperature. The Arrhenius plots were linear with both isoenzyme preparations up to 45 degrees C. Between 45 degrees C and 50 degrees C it appeared that this linear relationship no longer held, particularly with the liver tissue. When the activation energies were calculated both isoenzyme preparations exhibited several points of inflexion, in each case occuring at the same temperatures. These inflexions represent a change in the reaction kinetics, possibly a conformational change in the enzyme. The results also indicate that the LDH 1 and 2 isoenzymes are more efficient than LDH 4 and 5.