Temperature effects on key metabolic enzymes in Littorina saxatilis and L-obtusata from different latitudes and shore levels

Effects of temperature on activities of key metabolic enzymes, citrate synt hase (CS), NADP-dependent isocitrate dehydrogenase (NADP-IDH), aspartate am ino transferase (AAT), pyruvate kinase (PK) and phosphoenolpyruvate carboxy kinase (PEPCK), were studied in high and low shore Littorina saxatilis and in low shore L. obtusata from the temperate North Sea and the sub-arctic Wh ite Sea. It was found that adaptation of L. saxatilis and L. obtusata to li fe at different latitudes and/or shore levels involves constitutive changes in enzymatic activities, so that animals naturally adapted to (micro-) env ironments with lower mean temperatures (i.e. in the White Sea or at low sho re levels) tend to have higher enzyme activities as compared to their count erparts from warmer habitats (e.g. the North Sea or high shore levels, resp ectively). This suggests metabolic temperature compensation in different po pulations/subpopulational groups of this eurythermal species. Activities of all five studied enzymes were modulated by adaptations to high shore life in L. saxatilis, whereas only NADP-IDH, PK and PEPCK had different activiti es in animals from different latitudes. Adaptation to high shore life also involved an enhanced potential for anaerobic energy production via the succ inate pathway in Littorina spp. (measured as the ratio of PEPCK and PK acti vities), which increased in the order: low shore L. obtusata less than or e qual to low shore L. saxatilis less than or equal to high shore L. saxatili s. Possibly, an increased anaerobic capacity is advantageous for snails at high shore levels, where they may experience limited oxygen access during p rolonged air exposure. In contrast, the activation energies of the respecti ve enzymes did not reveal a consistent pattern of variation and were simila r in animals from different latitudes and shore levels. Arrhenius breakpoin t temperatures (ABT) of the studied enzymes characterising a change in the thermal properties of the protein were found well within the average highs of ambient temperatures (20-35 degreesC). Denaturation temperatures (T-d), indicating heat inactivation of the protein, were close to the environmenta l extremes experienced by L. saxatilis during summer low tide. These findin gs suggest that the metabolic machinery of this eurythermal species may fun ction close to its physiological limits during summer low tide, especially at high shore levels.