Inhibition of lactate dehydrogenase activity from Dirofilaria immitis by suramin

Lactate dehydrogenase from adult D. immitis was partially purified and characterized. The molecular weight of the enzyme was determined to be 130 000. The specific activity of the lactate dehydrogenase was 4.1 U/mg in the direction of NADH-oxidation and 0.38 U/mg protein in the direction of NAD-reduction. The Michaelis constants were determined to be 0.028 mM and 0.9 mM for NADH and NAD, respectively. Suramin was found to be a potent inhibitor of the lactate dehydrogenase activity. The inhibition constant was calculated to be 0.006 mM. The type of inhibition by suramin was competitive with respect to the cofactors. The Km-values for pyruvate and lactate were determined to be 0.3 mM and 11mM, respectively.