Kinetic properties and thermal stabilities of Geophagus brasiliensis skeletal muscle unfractionated malate dehydrogenase (MDH, EC 1.1.1.37) and its isolated isoforms were analyzed to examine a possible sMDH-B* locus duplication in a fixation process influenced by genetic drift. Two optimal pHs were detected: 7.5 for AB1 unfractionated muscle phenotype and its B1 isoform, and 8.0 for AB1B2 unfractionated muscle phenotype, A and B2 isoforms. While G. brasiliensis A isoform could be characterized as thermostable, the duplicated B isoform cannot be assumed as thermolabile. Km values for isolated B2 isoforms were 1.6 times lower than for B1. A duplication event in progress best explains the electrophoretic six-band pattern detected in G. brasiliensis, which would be caused by genetic drift.

A fim de explicar o padrão eletroforético de seis componentes detectado para a malato desidrogenase solúvel (MDH, EC 1.1.1.37) em 84% dos exemplares de G. brasiliensis analisados (Cichlidae, Perciformes), uma duplicação recente no loco sMDH-B* é sugerida. Diluições seriadas de Klebe realizadas com extratos de músculo esquelético mostraram para as subunidades B1 e B2 o mesmo ponto final visual sugerindo um padrão de expressão não divergente para esses genes duplicados. Uma vez que não existe evidência de poliploidia na família Cichlidae, é sugerido que a duplicação no loco sMDH-B* seja resultante de uma duplicação regional. Especificidade tissular, termoestabilidade e propriedades cinéticas mostraram-se similares para as isoformas B, em ambos os fenótipos detectados, sugerindo estarem esses sob a ação do mesmo gene regulador. Os resultados similares obtidos para os fenótipos de três (AB1) e seis (AB1B2) componentes aqui analisados não mostraram nenhum indicativo de vantagem adaptativa deste último sobre o primeiro, em região subtropical.

A recent locus duplication hypothesis for sMDH-B* was proposed to explain the complex electrophoretic pattern of six bands detected for the soluble form of malate dehydrogenase (MDH, EC 1.1.1.37) in 84% of the Geophagus brasiliensis (Cichlidae, Perciformes) analyzed (AB1B2 individuals). Klebe's serial dilutions were carried out in skeletal muscle extracts. B1 and B2 subunits had the same visual end-points, reflecting a nondivergent pattern for these B-duplicated genes. Since there is no evidence of polyploidy in the Cichlidae family, MDH-B* loci must have evolved from regional gene duplication. Tissue specificities, thermostability and kinetic tests resulted in similar responses from both B-isoforms, in both sMDH phenotypes, suggesting that these more recently duplicated loci underwent the same regulatory gene action. Similar results obtained with the two sMDH phenotypes did not show any indication of a six-banded specimen adaptive advantage in subtropical regions.