Kinetic analysis of duck epsilon-crystallin, a lens structural protein with lactate dehydrogenase activity

Biochemical characterization and kinetic analysis of epsilon-crystallin from the lenses of common ducks were undertaken to elucidate the enzyme mechanism of this unique crystallin with lactate dehydrogenase (LDH) activity. Despite the structural similarities between epsilon-crystallin and chicken heart LDH, differences in charge and kinetic properties were revealed by isoenzyme electrophoresis and kinetic studies. Bi-substrate kinetic analysis examined by initial-velocity and product-inhibition studies suggested a compulsory ordered Bi Bi sequential mechanism with NADH as the leading substrate followed by pyruvate. The products were released in the order L-lactate and NAD+. The catalysed reaction is shown to have a higher rate in the formation of L-lactate and NAD+. Substrate inhibition was observed at high concentrations of pyruvate and L-lactate for the forward and reverse reactions respectively. The substrate inhibition was presumably due to the formation of epsilon-crystallin-NAD(+)-pyruvate or epsilon-crystallin-NADH-L-lactate abortive ternary complexes, as suggested by the product-inhibition studies. The significance and the interrelationship of duck epsilon-crystallin with other well-known LDHs are discussed with special regard to its role as a structural protein with some enzymic function in lens metabolism.