The role of phosphoenolpyruvate carboxykinase in the anaerobic metabolism of the sea musselMytilus edulis L.

Abstract

The importance of PEP-carboxykinase and the utilization of aspartate in the anaerobic formation of succinate in the sea mussel was investigated in in vivo experiments by applying the metabolic inhibitors 3-mercaptopicolinate and aminooxyacetate, inhibitors of PEP-carboxykinase and transaminases, respectively. The posterior adductor muscle was analyzed separately from the remainder of the soft body (Rest). The following results were obtained: 1. In winter, succinate formation in the adductor muscle could be blocked by aminooxyacetate, whereas 3-mercaptopicolinate displayed no effect. This was studied for the 6 h following the onset of aerial exposure and for the period 12–18 h. In contrast, both inhibitors reduced the formation of succinate in the Rest. Therefore in adductor muscle all carbon for succinate formation must have been derived from aspartate, whereas for the Rest both aspartate and carbohydrate utilization (via PEP-carboxykinase) must have been involved. In summer, however, for adductor muscle a reduction of succinate formation and an increased aspartate utilization was observed after application of 3-mercaptopicolinate. The involvement of PEP-carboxykinase in the anaerobic formation of succinate in the adductor muscle appears therefore to be depending on the season. 2. In the adductor muscle extra strombine and octopine were formed when aspartate utilization was blocked by aminooxyacetate. In the Rest opines were no important end products. 3. Forced shell valve closure resulted, when compared to normal aerial exposure, in an increased utilization of aspartate and an increased accumulation of alanine and succinate. Only in the groups subjected to forced valve closure strombine, and to a lesser extent octopine, appeared to accumulate in substantial quantities. When anaerobiosis progresses, the rate at which the concentration changes of aspartate, alanine and succinate occur declines, but not that of strombine formation. 4. For the adductor muscle the decrease in aspartate exceeded the increase in succinate. The opposite was true for the Rest. In the adductor muscle only part of the utilized aspartate is used for succinate formation, whilst the other part in converted via transaminations and a decarboxylation into alanine according to the overall reaction aspartate →CO2+alanine. It is suggested that the latter conversion may be involved in anaerobic hydrogen transport across the mitochondrial inner membrane.