Effects of pH, ionic strength, and metabolic intermediates on the rates of heat inactivation of lactate dehydrogenase isozymes.

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Abstract
Reduced nicotinamide adenine dinucleotide (NADH), oxalacetate, malate, and fructose-1,6-diphosphate as well as alterations in pH and ionic strength significantly affected the rates of heat inactivation of LDH 1 [lactate dehydrogenase 1 from pig heart] and LDH 5 [lactate dehydrogenase 5 from rabbit muscle]. Oxalacetate and malate differentially protected LDH 5 from heat inactivation, whereas NADH and fructose-1,6-diphosphate protected both LDH 1 and LDH 5. In Rana catesbeiana, temperature changes in vivo produced alterations of liver LDH isozyme patterns; frog kept at 37[degree]C lost heat-labile isozymes (LDH 1 and LDH 2) conspicuous at lower temperatures. The possibilities that differential rates of catabolism play a role in regulating tissue izozyme patterns and that all osteric effectors may protect certain isozymes from catabolism are discussed.