A mathematical model describing the effect of temperature and substrate concentration on the activities of M 4 and H 4 lactate dehydrogenase from the big brown bat ( Eptesicus fuscus)

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The effect of temperature on the activities of M 4 and H 4 lactate dehydrogenases (LDH, EC 1.1.1.27) isolated from the big brown bat ( Eptesicus fuscus) was examined. Temperature effects were dependent on the concentrations of all four LDH substrates, pyruvate, lactate, NADH, and NAD. Arrhenius plots of In v i vs reciprocal of absolute temperature were linear for all but the lowest substrate concentrations. The slopes of these Arrhenius plots were used to calculate the temperature effect parameter (μ). Substrate-dependent temperature effects for M 4 and H 4 LDH were described by an equation for a rectangular hyperbola, μ = [E βS + E αK t] [K t + S] proposed by G. R. Harbison and J. R. Fisher (1974, Comp. Biochem. Physiol. 47B, 27–32) for adenosine deaminase. The parameters E α (μ at infinitely low substrate concentration), E β (μ at infinitely high substrate concentration), and K t (the concentration of substrate when μ = [E α + E β] 2 ) can be used to describe the temperature dependence of LDH activity at any substrate concentration and to compare the substrate-dependent temperature effects on the two isoenzymes. Significantly different E β and K t values for pyruvate-dependent temperature effects and different E β , E α , K t , and E β − E α (the range of possible μ values) for lactate-dependent temperature effects were found between M 4 and H 4 LDH isoenzymes. High lactate concentrations inhibited bat H 4 LDH activity to a greater degree at low temperatures than at high temperatures. Thus substrate inhibition plays an important role in the effect of temperature on the activity of H-type LDH at high lactate concentrations. Substrate-dependent temperature effects on bat LDH activity were the result of temperature effects on the apparent K m value of the respective substrate. Since both the apparent K m for pyruvate and the K i for the competitive inhibitor oxamate decreased with decreasing temperature, the substrate-dependent temperature effects observed for pyruvate probably resulted from an increased affinity between pyruvate and the LDH-NADH complex with decreasing temperature.