Lactate Dehydrogenase Isozymes of Salmonid Fish EVIDENCE FOR UNIQUE AND RAPID FUNCTIONAL DIVERGENCE OF DUPLICATED H, 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 Hq lactate dehydrogenase isozyme of higher vertebrates (BAILEY, G. S., AND WILSON, A. C. (1968) J. Biol. Chem. 243, 5843). The H4 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 Mq isozyme of salmon. The two proteins H4 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 Hq and H’, isozymes were shown to be about as different from each other as are the H4 lactate dehydrogenases of chicken and duck. This suggests that the gene duplication event in salmon which gave rise to two Ldh H genes occurred approximately 80 to 100 million years ago. The H’, lactate dehydrogenase which has arisen from this duplication in salmon is shown to be somewhat intermediate between H4 and M4 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 Hlq isozyme is almost exactly intermediate between Hq and Mq in its resistance to product inhibition by lactate, the catalytic parameter suggested to be of major functional importance to Mq 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 Mq isozyme occurs in very few tissues in detectable levels. It is the Hq and H’,, rather than H, and Mq, which occur in independ-