Lactate dehydrogenase isozymes of salmonid fish. Evidence for unique and rapid functional divergence of duplicated H-4 lactate dehydrogenases.
Salmonid fish, as a result of total genome duplication, have two genes, Ldh H and Ldh H', coding for polypeptides H and H', respectively, both of which have been shown in their tetrameric forms to be immunologically related to the classical H-4 lactate dehydrogenase isozyme of higher vertebrates (Bailey, G. S., and Wilson, A. C. (1968) J. Biol. Chem. 243,5843). The H-4 and H'-4 isozymes have now been highly purified from quinnat salmon, and their chemical, physical, immunological, and catalytic properties examined, and compared to the M-4 isozyme of salmon. The two proteins H-4 and H'-4 are shown to be very similar in amino acid composition, but significant differences in a few residues suggest differences in amino acid sequences. This suggestion was born out by quantitative immunological experiments in which the H-4 and H'-4 isozymes were shown to be about as different from each other as are the H-4 lactate dehydrogenases of chicken and duck. This suggests that the gene duplication event in salmon which give rise to two Ldh H genes occurred approximately 80 to 100 million years ago. The H'-4 lactate dehydrogenase which has risen from this duplication in salmon is shown to be somewhat intermediate between H-4 and M-4 in thermal stability, and in all catalytic properties examined, including substrate optima, Michaelis constants, and susceptibility to inhibition by high levels of substrate. In particular the H'-4 isozyme is almost exactly intermediate between H-4 and M-4 in its resistance to product inhibition by lactate, the catalytic parameter suggested to be of major functional importance to M-4 lactate dehydrogenase isozymes (Stambaugh, R., and Post D. (1966) J. Biol. Chem. 241,1462). Further, tissue distribution of these isozymes in salmon and trout are shown to be unusual. The M-4 isozyme salmon and trout are shown to be unusual. The M-4 isozyme occurs in very few tissues in detectable levels. It is the H-4 and H'-4 rather than H-4 and M-4, which occur in independently variable but significant levels in most tissues examined. Thus the H'-4 isozyme, despite its very close structural similarity to H-4 appears to possess functional properties which are different from either H-4 or M-4 in salmon, and some properties are midway between the two. This finding, together with the unusual tissue distribution of these isozymes, suggests that salmon with H'-4 lactate dehydrogenase is evolving to function catalytically in the absence of a balanced H-4-M-4 isozyme complement in most tissues. This balance seems to be met in most tissues by combinations of H-4 and H'-4,