Urea-requiring lactate dehydrogenases of marine elasmobranch fishes

Summary The kinetic properties — apparentK_m of pyruvate, pyruvate inhibition pattern, and maximal velocity — of M₄ (skeletal muscle) lactate dehydrogenases of marine elasmobranch fishes resemble those of the homologous lactate dehydrogenases of non-elasmobranchs only when physiological concentrations of urea (approximately 400 mM) are present in the assay medium. Urea increases the apparentK_m of pyruvate to values typical of other vertebrates (Fig. 2), and reduces pyruvate inhibition to levels seen with other M₄-lactate dehydrogenases (Fig. 3). Urea reduces the activation enthalpy of the reaction, and increasesV_max at physiological temperatures (Fig. 4).The M₄-lactate dehydrogenase of the freshwater elasmobranch,Potamotrygon sp., resembles a teleost lactate dehydrogenase, i.e., although it is sensitive to urea, it does not require the presence of urea for the establishment of optimal kinetic properties.