Soybean L-(+)-lactate dehydrogenases: purification, characterization, and resolution of subunit structure

Lactate dehydrogenase (LDH) (EC 1.1.1.27) from soybean (Glycine max) was purified 2360-fold to homogeneity using ion-exchange, hydroxyapatite, affinity, and hydrophobic chromatographies. The molecular weight of the holoenzyme is 150,000 +/- 5000. Two-dimensional (isoelectrofocusing-sodium dodecyl sulfate) gel electrophoresis reveals two polypeptides subunits of 5.9 and 6.5 pI and of 36,000 +/- 1000 and 37,000 +/- 1000 Mr, respectively. Nondissociating electrophoresis and isoelectric focusing of LDH resolved five tetrameric isoenzymes with pI's between 6.0 and 6.5. The data suggest that these LDH isoenzymes are derived from random association of the products of two different, but most probably related, genes. Kinetic measurements revealed substrate inhibition at high concentrations of lactic acid and biphasic kinetics with NAD.