Properties of human testis-specific lactate dehydrogenase expressed from Escherichia coli

The cDNA encoding the C₄ isoenzyme of lactate dehydrogenase (LDH-C₄) was engineered for expression in Escherichia coli. The Ldh-c open reading frame was constructed as a cassette for production of the native protein. The modified Ldh-c cDNA was subcloned into the prokaryotic expression vector pKK223-3. Transformed E. coli cells were grown to mid-exponential phase, and induced with isopropyl β-D-thiogalactopyranoside for positive regulation of the tac promoter. Induced cells expressed the 35 kDa subunit,which spontaneously formed the enzymically active 140 kDa tetramer. Human LDH-C₄ was purified over 200-fold from litre cultures of cells by AMP and oxamate affinity chromatography to a specific activity of 106 units/mg. The enzyme was inhibited by pyruvate concentrations above 0.3 mM, had a K(m) for pyruvate of 0.03 mM, a turnover number (nmol of NADH oxidized/mol of LDH-C₄ per min at 25°C) of 14000 and was heat-stable.