Multiple forms of supernatant malate dehydrogenase in salmonid fishes.

Salmon generally possess three forms of supernatant malate dehydrogenase whose subunit compositions are indicated by the formulas AA, AB, and BB. Each of the homodimers (AA and BB) has now been purified to homogeneity. The two enzymes are similar in molecular size and in catalytic properties to the supernatant malate dehydrogenases of higher vertebrates; both are catalytically distinct from the mitochondrial form of this enzyme in salmon as well as in higher vertebrates. The two enzymes differ significantly in amino acid composition. The compositional differences are small enough to suggest that the amino acid sequences of A and B subunits are related. Such a relationship is supported by immunological experiments conducted with rabbit antisera prepared against the purified enzymes. These findings suggest that the A and B subunits are the products of duplicate genes which have undergone limited evolutionary divergence. Salmon and trout populations were examined by electrophoresis for malate dehydrogenase variants. The B-type subunit was found to exist in two common forms, B and B’. Most king salmon were found to produce only the B form, whereas most rainbow trout possess the B’ form. A few salmon and trout were observed to be heterozygotes producing both the B and B’ subunits. By chromatographic and densitometric determination, most rainbow trout heterozygotes possessed a distinctly asymmetrical 1: 6:9 ratio of the three enzymes, BB, BB’, B’B’; in contrast, king salmon heterozygotes possessed a 9: 6: 1 ratio of BB, BB’, and B’B’. The genetic basis of these peculiar ratios was investigated by matings of heterozygous and homozygous salmon. The offspring were grown for 4 months and tested electrophoretitally for the presence and ratio of B and B’ subunits. The results demonstrate that salmon possess not one but two genetic