Facultative anaerobiosis in molluscs

Abstract

The glycolytic fermentation of molluscs is rather complex. Multiple end products accumulate (lactate, alanine, octopine, succinate, propionate, acetate and CO2), which are partly formed in the cytoplasm and partly in the mitochondrion. Various schemes have been presented to account for these end products as well as for the maintenance of the redox balance. With respect to the role of alanine there are two opinions: (1) alanine accumulation is continuous and is essential for the generation of the mitochondrial NADH required in the reduction of fumarate and (2) succinate and alanine (initial end products) accumulate in different compartments and their accumulation occurs independently. Both statements are evaluated in the light of the latest experimental observations including the regulatory properties at the phosphoenolpyruvate branchpoint and the effect of pH and 'energy charge'. For nervous tissue the function of oxygen can be replaced by the lipochrome pigment, which enables carbohydrates to be totally oxidized to CO2 and water. The simultaneous mobilization of carbohydrates and amino acids is not supported by the experimental data. Various advantages of the glycolytic fermentation in molluscs as compared with classical glycolysis in skeletal muscle are discussed.