Abstract

MDH1, isoenzyme of malate dehydrogenase from blood stream Trypanosoma vivax was characterized. The enzyme was active over a broad pH and temperature range with optimal values of 5.0 and 35^oC respectively. The energy of activation was 20.58kj/mol and the pKa values were 6.8 and 7.8 implicating ionizable groups at the catalytic site. Kinetic studies conducted in the direction of oxaloacetate reduction gave K_M of 0.56 and 0.3 mM and V_max of 4.6 and 3.2 µmol./ min/mg for oxaloacetate and NADH respectively. Similarly, in the reverse reaction, malate had K_M of 0.16 Mm and V_max of 57 µmol./min/mg, and K_M and V_max of NAD⁺ were 0.1 5 mM and 48 µmol./min/mg. The enzyme was inhibited competitively with NAD⁺ as a product inhibitor and uncompetitive with malate. The product inhibition studies suggest bi-bi ordered sequential mechanism of catalysis. Some TCA cycle intermediates also inhibited to different extent the activity of the enzyme.

Key words: Malate dehydrogenase, Trypanosoma vivax, isoenzyme.