PYRUVATE METABOLISM IN THE LOBSTER NERVE AS AFFECTED BY THE PARTIAL PRESSURE OF CARBON DIOXIDE: OBSERVATIONS ON THE SYNTHESIS OF ACETYLCHOLINE AND ON METABOLIC COMPARTMENTATION

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Abstract
Abstract— In the lobster nerve the fixation of CO, at various levels of pCO2 was studied by the incorporation of [l‐14C]pyruvate. Incorporation of 14C was solely dependent on CO2 fixation since the C‐1 was decarboxylated in the formation of acetyl‐CoA. Paired‐nerve studies with [2‐14C]pyruvate afforded a study of pyruvate metabolism in the lobster nerve. [I14C]Pyruvate was incorporated to nearly the same extent at all levels of pCO2 including zero pCO2, a finding that suggested metabolic recycling of CO2. The magnitude of the metabolic recycling of C‐1 of pyruvate or pyruvate dismutation was estimated to be nearly 20 per cent of total CO2 fixation. Re‐evaluation of the relative contributions of the CO2 fixation. and acetyl‐CoA pathways on the basis of more extensive data gave a ratio of 2:3.The pCO2 affected synthesis of ACh and the level of citrate. With increasing pCO2, the specific radioactivity of ACh decreased much more than the content of ACh. The decrease in the specific radioactivity of ACh but not that of citrate further suggested metabolic compartmentation. The implication of these findings is discussed.Alanine functioned as a metabolic sink for the incorporated pyruvate. Pyruvate levels were estimated to be approximately 0.1 nmol/mg of protein.