Lactate dehydrogenase (LDH) was isolated from pectoral or flight muscle, liver and heart of hibernating and normothermic bats, Myotis lucifugus. 2. Activities at high substrate concentrations and the interactions between substrate and enzyme were studied to evaluate possible adaptations associated with the extended period of depressed metabolism of this bat. 3. Activity levels in all tissues decreased with hibernation, although the magnitude was dependent upon both temperature and substrate concentration. In general, the extent of decrease was in the order pectoral muscle greater than liver much greater than heart. 4. No correlation was found between the temperature sensitivity of the tissue LDHs as estimated by Ea-values, and the physiological state. 5. However, affinity parameters as estimated by Km-values were less temperature sensitive in hibernator tissues in contrast to the marked perturbations exhibited at temperature extremes in the normotherm enzymes. 6. It is suggested that changes in subunit quantities are in part responsible for these kinetic differences, and not major qualitative isozyme changes. 7. The net result of these effects is a reduced carbon flux to lactate during hibernation, but the maintenance of a potential for LDH catalyses during arousal.
