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.

Adaptative features of ectothermic enzymes. III--Studies on phosphoglucose isomerase (PGI) from five species of tropical fishes of the superorder Ostariophysi.

1. The electrokinetic and thermostability properties of phosphoglucose isomerase (PGI) in five species of tropical fish (Ostariophysi)-Leporinus friderici, L. silverstrii, Schizodon nasuttus, Hypostomus sp and Pimelodus maculatus--have been studied in order to study the adaptative nature of protein heterogeneity found in ectotherms. 2. Unlike most diploid fishes, the PGI of these species seemed to be encoded by three, four or five loci. 3. The subunits encoded by these loci occurred at different levels in the different tissues and organs analyzed. 4. Genetic variants at the three PGI loci were detected in two species of the family Anostomidae (L. friderici and S. nasuttus). In the family Pimelodidae, P. Maculatus showed a genetic variant as the Pgi-1 locus. 5. The product of these loci could be separated in three PGI regions based on their electrophoretic mobility, tissue distribution and thermostability properties. 6. The more anodal region (including isozymes and allozymes) was more thermolabile then the less anodal one, which is predominant in a single tissue-skeletal muscle.