Adaptative features of ectothermic enzymes--IV. Studies on malate dehydrogenase of Astyanax fasciatus (Characidae) from Lobo Reservoir (Såo Carlos, Såo Paulo, Brasil).

1. Skeletal muscle and heart supernatant malate dehydrogenase (s-MDH) from a subtropical fish, Astyanax fasciatus consists of three electrophoretically anodal bands. Each band is a dimer (AA, AB and BB) and two loci are active. 2. In A. fasciatus tissue extracts, A and B subunits are present at differing quantitative levels and their activities are almost season-independent. However, the relative activity of each homodimer in relation to total s-MDH estimated by densitometry of gels or of each homodimer purified by chromatography varies with temperature. The more anodic homodimer is thermolabile and the less anodic one is thermostable. 3. The pH optimum of s-MDH is 7.5, of AA is 6.5 and of BB is 7.8. 4. The BB isozyme is more sensitive to high concentrations of substrate and has a Km temperature-independent. The AA isozyme is not inhibited by high concentrations of oxaloacetate and shows a Km temperature-dependent with a fourteenfold increase between 20 degrees and 40 degrees C.