The pyruvate branchpoint in the anaerobic energy metabolism of the jumping cockle Cardium tuberculatum L.: D-lactate formation during environmental anaerobiosis versus octopine formation during exercise.

The cockle Cardium tuberculatum responds with a typical escape movement (jumping by foot contractions) when touched by a starfish. In addition, this species can survive anoxic conditions for up to 20 h at 22 degrees C. The maximum activities of various enzymes involved in energy metabolism were determined in foot, adductor muscle and gills. Three enzymes with pyruvate reductase activity (lactate-, octopine-, alanopine/strombine dehydrogenase) can possibly serve as the terminal step of anaerobic glycolysis. On the whole the activities of enzymes involved in aerobic metabolism are low. In whole specimens of C. tuberculatum the metabolic response to anoxic incubation, exercise and exercise following anoxia were investigated. During 15 h of incubation in oxygen-free sea water no significant change in the energy charge is observed. The levels of arginine-phosphate fall to 50% of the value in resting animals with a concomitant increase in arginine concentration, but virtually no formation of octopine occurs. D-lactate and alanine are the main end products of anaerobic glycolysis and accumulate to a similar extent during anoxic incubation. While D-lactate levels increase linearly with time, L-alanine exhibits a 2-h delay before concentrations begin to rise. There is little formation of succinate and no formation of propionate and acetate. Despite the remarkably high activities of the enzymes alanopine and strombine dehydrogenase only negligible amounts of alanopine are found after anoxia and no production of strombine occurs. Exercise results in a depletion of the arginine-phosphate stores, liberation of arginine and formation of octopine. In whole animals as well as in foot muscle these changes are closely correlated to the number of jumps performed. No accumulation of D-lactate, alanine and succinate was detected. Exercise after an anoxic preincubation period of 6 h results in a further depletion of the already diminished arginine-phosphate stores and formation of octopine as the only end product. During anoxia the contribution of glycolysis to the required energy is enhanced from 44% (after 2 h) to 86% (after 15 h). During exercise glycolysis contributes 36% of the energy demand but only 16% during exercise after anaerobic preincubation. Thus the direction taken at the pyruvate branchpoint can be predicted on the basis of the glycolytic flux. Octopine formation only occurs when the glycolytic flux is enhanced (during exercise at least 40-fold). At low glycolytic flux (60% reduction during anoxia) pyruvate is channelled to form D-lactate and alanine.