Thermostable lactate dehydrogenases (EC 1 . 1 . 1 .27) were purified to homogeneity from Clostridium thermohydrosulfuricum 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 x lo3, but slightly different subunits with M , values about 37 x lo3. LDHL dissociated into subunits more readily. The isoelectric points were 5.0 for LDHL and 5-2 for LDHE. The catalytic activity of LDHE had an almost absolute requirement for fructose 1,6-bisphosphate (FBP) at all temperatures (22-fold activation with K I l 2 12 p~-F B P at 65 "C, pH 6.0). LDHL was activated by FBP only at temperatures over 40 "C (5-fold activation with K,12 80 ~M -F B P 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 LDHL for pyruvate. At 50 p~-F B P both enzymes showed a positive co-operative dependence on NADH.

No abstract