GENETIC-VARIATION AND RELATIVE CATALYTIC EFFICIENCIES - LACTATE DEHYDROGENASE-B ALLOZYMES OF FUNDULUS-HETEROCLITUS

In order to evaluate whether functional differences exist between allelic variants of a B type lactate dehydrogenase (LDH; L-lactate:NAD+oxidoreductase, EC 1.1.1.27) in the teleost fish Fundulus heteroclitus (Linnaeus), the kinetic properties of pyruvate reduction were examined. While the pH dependence and the temperature dependence for maximal catalysis were indistinguishable among the allozymes, reaction velocities at low pyruvate concentrations were significantly different. At pH values below 8.00, the LDH-B(b)B(b) allozyme showed a greater reaction rate at lower temperatures (e.g., 10°C) than LDH-B(a)B(a). The phenomenon was reversed at higher temperatures (e.g., >25°C) for pH values between 6.50 and 7.00. The rates for the heterozygous phenotype, LDH-B(a)B(b), were not the arithmetic average of the two homotetrameric allozymes. When reaction rates were compared at constant relative alkalinity, that is a constant [OH-]/[H+] ratio, the findings were similar. The differences in the temperature dependence and the pH dependence for pyruvate reduction found between the LDH-B allozymes may reflect a selective adaptation and help explain the geographical variation in the Ldh-B gene frequencies of F. heteroclitus.