Der einfluß der temperatur auf die substrat-affinität der laktat-dehydrogenase aus fischen

If the activity of lactate-dehydrogenase (LDH) at small pyruvate concentrations (e.g. 20μM), as found in vivo, is measured, no temperature-dependence of the reaction-velocity is noted. This finding implies, for a poikilothermic animal (e.g. the fish Rhodeus amarus), that in physiological substrate concentrations, the velocity of enzymatic reaction is temperature-independent. In the LDH of R. amarus acclimated to 10° or 20°C, the enzyme-substrate-affinity, as judged by the Michaelis-constant (K _M), changes inversely with temperature. In contrast to R. amarus, K _M-values of Idus idus have a minimum temperature range identical to the adaptation temperature (AT). The momentary AT (e.g. 10°, 20°C) induces the disposition of an enzyme with greatest substrate-affinity at that temperature, which coincides with the AT. Estimating the activation energy (′E _a=μ) through the Arrhenius-plot, ′E _a for LDH of I. idus amounts to 8.2 kcal/mol in the experimental temperature (ET) range of 5° to 15°C. For ETs above 15°C, a value of ′E _a=12 kcal/mol is found. LDH of I. idus of 20°C, however, shows a smaller value of ′E _a=7.8 kcal/mol beyond ET=15°C; below ET=15°C, ′E _a=10 kcal/mol. If the AT of R. amarus of 10°C is raised to 25°C, and the K _M of LDH for substrate pyruvate and cosubstrate NADH (ET always 25°C) is measured, a dampened oscillation-like curve will be found, depending on time.