ECOPHYSIOLOGICAL STUDIES ON CITRATE SYNTHASE .2. ENZYME REGULATION OF SELECTED CRUSTACEANS WITH REGARD TO LIFE-STYLE AND THE CLIMATIC ZONE

Citrate synthase is a regulatory enzyme of the energy metabolism pathway controlling the citric acid cycle. It was studied in order to determine modes of enzyme regulation with regard to the life-style of the investigated species. Citrate synthase from crustaceans with different life-styles were compared: the pelagic euphausiids Euphausia superba from the Antarctic and Meganyctiphanes norvegica from the Scandinavian Kattegat and the Mediterranean were compared to the benthic isopods Serolis polita from the Antarctic and Idotea baltica from the Baltic. Citrate synthase was partly purified chromatographically and the influence of adenosine 5'-triphosphate on enzyme activity was examined. Mechanisms of inhibition and inhibitor constants were determined. Two different mechanisms of enzyme regulation by ATP were found. Citrate synthase from isopods was only competitively inhibited. while citrate synthase from euphausiids showed not only competitive inhibition but also activation by low concentrations of ATP. This activation is equivalent to the reversed mechanism of uncompetitive inhibition. The ecophysiological relevances of the coupling of these mechanisms are discussed. The degree of competitive inhibition was different in the two groups of investigated crustaceans. Inhibitor constants were similar within the euphausiids but not in isopods. which showed higher or lower inhibition depending on the climatic zone: the colder the ambient temperature the lower the ATP inhibition. A possible mechanism of temperature adaptation through effects of varying inhibition constants is concluded.