Alternatives to lactic acid: Possible advantages

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Abstract
Many groups of invertebrates use metabolic strategies that do not lead to the accumulation of lactate during periods of hypoxia or anoxia. These animals may accumulate some combination of the following: alanine, octopine, alanopine, strombine, acetate, propionate, 2‐methylbutyrate, 2‐meth‐ylvalerate, and succinate. An organism may derive a benefit from production of an alternative end product when the yield of ATP per mole of glucose 6‐phosphate is greater than that found in lactate production. Formations of succinate, propionate, 2‐methylbutyrate, and 2‐methylvalerate have been shown to yield more ATP in a few species. This analysis can be extended to the formation of alanine, as this is accompanied by the conversion of aspartate to succinate.Formations of octopine, alanopine, or strombine apparently do not increase the yield of ATP per mole of glucose 6‐phosphate, because the enzymes forming these compounds are functionally analogous to lactate dehydrogenase. A potential advantage of producing these compounds might be maintenance of a constant intracellular osmotic pressure during periods of anoxia. The significance of this is uncertain, because if lactate were to accumulate, the expected change in osmotic pressure appears to be trivial. Another possible advantage of accumulating octopine, alanopine, or strombine, would be the ability to maintain a lower NADH/NAD+ ratio as compared with the accumulation of lactate. This might assist the organism in maintaining a high rate of glycolysis by reducing the inhibition of glyceraldehyde 3‐phosphate dehydrogenase by NADH. Other possibilities are smaller perturbations of the acid–base balance of the cell, or producing a compound that has less effect on the catalytic and regulatory properties of enzymes in the cell.

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