Temperature dependence of intracellular pH: Its role in the conservation of pyruvate apparentKm values of vertebrate lactate dehydrogenases

Summary When apparent Michaelis constants (K_m's) for pyruvate of M₄-lactate dehydrogenases from differently thermally adapted vertebrates are measured at the species' normal cell temperatures, a marked degree of conservation inK_m is observed, but only when the pH of the assay medium is varied in the manner in which intracellular pH varies with temperature in most animals (Fig. 2).K_m measurements performed at a constant pH do not yield this high degree of interspecific conservation inK_m (Figs. 2 and 3).The temperature dependence of intracellular pH preserves the charge states of imidazoles of protein histidines during temperature transitions. Thus under intracellular conditions the ionization state of the active site histidine of LDH will be independent of temperature, reducing the temperature dependence of pyruvate binding. This effect appears important in the contexts of short-term temperature variation experienced by an individual ectotherm and of long-term, evolutionary temperature changes important in speciation processes.These findings emphasize the importance of utilizing biologically realistic pH values in enzyme studies if major adaptive trends are to be observed.