Kinetic Characterization of the Lactate Dehydrogenase (LDH-B4) Allozymes of Fundulus Heteroclitus

In order to evaluate whether functional differences exist between allelic variants (allozymes) of the B-type lactate dehydrogenase (L-lactate:NAD+ oxidoreductase, EC 1.1.1.27) in the fish Fundulus heteroclitus, the kinetic properties were examined. Differences in product inhibition between allelic isozymes were observed. LDH-Bb4 was found to be inhibited by lower concentrations of both lactate and pyruvate than either LDH-Ba/Bb or LDH-Ba4. While the pH and temperature dependence for maximal catalysis (Vmax) in both the forward and reverse directions were indistinguishable among the allozymes, reaction velocities at low substrate concentrations (Vmax/Km) were significantly different. These differences could be explained by LDH-Bb4 having a greater substrate affinity than LDH-Ba4 at cold temperatures while the reverse was true at elevated temperatures. At a given pH, LDH-Bb4 showed a greater reaction rate at lower temperatures (e.g. 10 degrees C) than LDH-Ba4. The phenomenon was reversed at higher temperatures (e.g. greater than 35 degrees C). When reaction rates were compared at constant relative alkalinity, that is, a constant [OH-]/[H+] ratio, similar findings were obtained. The response for the heterozygous allozyme, LDH-Ba/Bb, was not the arithmetic average of the two homotetrameric allozymes. The biological significance of these allozymic differences is discussed.