DIFFERENTIAL SURVIVAL OF VENUS-GALLINA AND SCAPHARCA-INAEQUIVALVIS DURING ANOXIC STRESS - COVALENT MODIFICATION OF PHOSPHOFRUCTOKINASE AND GLYCOGEN-PHOSPHORYLASE DURING ANOXIA

Biochemical mechanisms underlying anaerobiosis were assessed in two Mediterranean bivalve species, Scapharca inaequivalvis and Venus gallina, with widely differeing tolerances for oxygen lack. These species displayed LT50 values for anoxic survival at 17-18-degrees-C of 17 and 4 d, respectively. Succinate and alanine were the major products of 24 h anaerobic metabolism in both species but only S. inaequivalvis further metabolized succinate to propionate. Both species reduced metabolic rate while anoxic but metabolic arrest was more pronounced in S. inaequivalvis. Calculated ATP turnover rate (MATP) during exposure to N2-bubbled seawater was only 4.51% of the aerobic rate in S. inaequivalvis but was 12.68% in V. gallina. To counteract a greater load of acid end products, V. gallina foot showed a significantly greater buffering capacity, 23.38 +/- 0.20 slykes, compared to 19.6 +/- 0.79 slykes in S. inaequivalvis. The two species also differed distinctly in the enzymatic regulation of anaerobiosis. In V. gallina anoxia exposure caused only a small change in PFK kinetic parameters (a decrease in K(a) AMP) and had no effect on glycogen phosphorylase. By contrast, S. inaequivalvis foot showed a strong modification of enzyme properties in anoxia. The percentage of glycogen phosphorylase in the a form dropped significantly only in S. inaequivalvis. Other changes included alterations in the properties of PFK leading to a less active enzyme form in anoxia. Compared to the aerobic enzyme form, PFK from anoxic foot showed a reduced affinity for fructose-6-P (K(m) increased 2.4-fold), greater inhibition by ATP (I50 decreased 6.8-fold), and an increase in sensitivity to AMP activation (K(a) decreased by 50%). These enzyme changes appear to be key to a glycolytic rate depression during anaerobiosis in S. inaequivalvis foot muscle.