Alcoholic Fermentation in Multicellular Organisms

Abstract

1. Ethanol is an end product of anaerobic metabolism in a surprisingly large variety of multicellular organisms. These include Angiosperms, Platyhelminthes, Aschelminthes, Acanthocephala, Arthropoda, and Vertebrata. 2. Ethanol formation proceeds in two steps (via pyruvate decarboxylase and alcohol dehydrogenase). 3. Pyruvate decarboxylase of plants is located in the cytosol, whereas that of animals is intramitochondrial and probably part of the pyruvate dehydrogenase complex. 4. Alcohol dehydrogenases of facultative anaerobes are NAD-rather than NADP-dependent, with the enzyme in the parasitic worm Moliniformis dubius as the only known exception. 5. Regulation at the pyruvate branch point in plants seems to involve three different mechanisms: (i) increases of pyruvate decarboxylase and alcohol dehydrogenase activity due to increased gene expression, (ii) control of pyruvate decarboxylase by the intracellular pH and the cytoplasmic NADH/NAD⁺ ratio, and (iii) inhibition of lactate dehydrogenase by ATP at low pH values. There is no evidence for increased expression of the pyruvate decarboxylase and alcohol dehydrogenase genes of animals during anoxia. Here, pyruvate decarboxylase is controlled by the intracellular pH and the mitochondrial phosphorylation potential, whereas the ethanol/lactate production ratio is dependent on the magnitude of the glycolytic flux. 6. The ethanol route has significant advantages (minimal acidosis, avoidance of osmotic problems, and endproduct inhibition of the glycolytic chain) but also has disadvantages (relatively low energy yield, loss of carbobydrate carbon). This may explain why only a few animals produce ethanol during anoxia, in contrast to plants, which generally respond to reduced O₂ availability by alcoholic fermentation.