SUMMARY: Thermostable lactate dehydrogenases (EC were purified to homogeneity from cells grown on starch and producing mainly ethanol (LDH) and from cells grown on sucrose and producing mainly lactic acid (LDHJ, and were found to be distinct isoenzymes. The two enzymes both had native M, values close to 145 × 10, but slightly different subunits with M, values about 37 × 10. LDHL dissociated into subunits more readily. The isoelectric points were 5.0 for LDH and 5.2 for LDH. The catalytic activity of LDH had an almost absolute requirement for fructose 1,6-bisphosphate (FBP) at all temperatures (22-fold activation with K1/2 12 μM-FBP at 65°C, pH 6.0). LDH was activated by FBP only at temperatures over 40°C (5-fold activation with K1/2 80 μM-FBP at 65°C, pH 6.0). For both enzymes the optimum temperature for pyruvate reduction in the presence of 1 mM-FBP was 70°C and the pH optimum at 65°C was sharp and at 5.5-6.0. FBP lowered the apparent K of LDH for pyruvate. At 50μM-FBP both enzymes showed a positive co-operative dependence on NADH.


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