REGULATION BY SALT OF ACTIVITY OF CITRATE SYNTHASES FROM OSMOREGULATORS AND OSMOCONFORMERS*

Abstract

AbstractCitrate synthases from an osmoregulator and 2 osmoconforming animals were purified and studied with respect to interaction with salt levels. Catalytic activity of the shrimp (osmoregulator) citrate synthase was markedly affected by buffer concentrations, and its apparent Km with respect to acetyl coenzyme A was increased almost 60‐fold by increased concentration of salt. The osmoconformer citrate synthases showed slight effects of concentration of salt: the apparent Km of the sea anemone citrate synthase was increased fractionally by salt and about 6‐fold for citrate synthase of the oyster.Citrate synthase from the osmoregulator shrimp had the conventional hyperbolic kinetics of saturation by acetyl coenzyme A in the presence of moderate concentrations of any salt, but the sea anemone and oyster citrate synthases had sigmoid kinetics in the same saline environment. The oyster was shown to regulate the levels of magnesium, potassium, and chloride in its tissues.The ecoadaptive significance of these findings is discussed and the generally standing concern of investigators is supported that biologically and physiologically realistic interpretations of enzyme regulation by ionic strength and any other factors can be derived only from studies of enzyme activities in situ.