Enzymatic and thermodynamic profiles of a heterotetramer lactate dehydrogenase isozyme in swine.

Lactate dehydrogenase (LDH) is a glycolytic enzyme that catalyzes the final step of glycolysis and produces NAD. In somatic cells, LDH forms homotetramers and heterotetramers that are encoded by two different genes: LDHA (skeletal muscle type, M) and LDHB (heart type, H). Analysis of LDH isozymes is important for understanding the physiological role of homotetramers and heterotetramers and for optimizing inhibition of their enzymatic activity as it may result in distinct effects. Previously, we reported that hydroxychloroquine (HCQ) inhibited LDH activity, but we did not examine isozyme specificity. In the present study, we isolated heterotetrameric LDH (HM) from swine brain, determined its kinetic and thermodynamic properties, and examined the effect of HCQ on its activity compared to homotetrameric LDH isozymes. We show that: (1) the K values for HM-mediated catalysis of pyruvate or lactate were intermediate compared to those for the homotetrameric isozymes, M and H whereas the V values were similar; (2) the K and V values for HM-mediated catalysis of NADH were not significantly different among LDH isozymes; (3) the values for activation energy and van't Hoff enthalpy changes for pyruvate reduction of HM were intermediate compared to those for the homotetrameric isozymes; (4) the temperature for half residual activity of HM was closer to that for M than for H. We also show that HCQ had different affinities for various LDH isozymes.