RESUMO
L(+)lactate dehydrogenase (LDH) from the electric organ of Electrophorus electricus (L.) was purified by ammonium sulfate precipitation, ion-exchange chromatography on DEAE-cellulose and gel filtration on Sephadex G-200. Purified LDH was homogeneous when examined by polyacrylamide gel electrophoresis under nondenaturing conditions. Both LDH activity and protein were demonstrable in the same band. The mobility of the LDH-5 isozyme is characteristic of the muscle type enzyme. Isoelectric focusing showed a single molecular species of pIO 6.5 +/- 0.4. The apparent molecular weight was 140,000 (+/- 10%) on the basis of gel filtration of Sephadex G-200. The effect of organic acids on the enzyme activity towards pyruvate (NADH) and lactate (NAD) was determined spectrophotometrically at 340 nm. Sodium oxamate behaved as a mixed inhibitor when lactate (NAD) was the substrate, whereas ethyl oxamate was an uncompetitive inhibitor. Both the sodium salt and the ester of oxamic acid were competitive inhibitors when pyruvate (NADH) was the substrate.