Electrophoretic thermostability tests of soluble malate dehydrogenases (sMDH) isozymes in tissue extracts of 21 subtropical fish belonging to the orders Characiformes, Siluriformes and Perciformes showed three distinct results. The first, characterized by thermal stability of the slowest-migrating band or A-isoform, was detected in 52% of all species. The second, exhibited in 29% of the species analyzed, had a bidirectionally divergent pattern of their sMDH locus expression, and was characterized by a nondivergent thermostability pattern of both sMDH-A* and B*. In the third category, obtained in 19% of the species studied (the four Siluriformes species), thermostability of the fastest-migrating bands, or B-isoforms, was observed. Comparison of the effects of habitat temperature on the activity of paralogous and orthologous isoforms in tissue extracts of two of these species with different thermostability properties (Leporinus friderici - thermostable sMDH-A*, and Pimelodus maculatus - reverse thermostability properties or reverse electrophoretic pattern), collected during winter and summer months, showed that A and B subunits were present at different quantitative levels and their activities were nearly season independent. Differences in susceptibility to temperature (50°C) of both sMDH loci from tissue extracts of these species were found. In P. maculatus, these susceptibilities helped strengthen one of the hypotheses: the reverse thermostability pattern, where the fastest-migrating band or the B-isoform was the thermostable sMDH. Thus, temperature differences among orthologous homologues of sMDH seem to have occurred in these acclimatized species, where the fastest-migrating band, usually muscle specific and thermolabile in most teleosts, appeared in P. maculatus as the thermostable isoform.
