The Metabolic Systems Involved in Dissimilation of Carbohydrate Reserves in Bakers' Yeast

Evidence is presented showing that the dissimilation of carbohydrate reserves in two strains of bakers' yeast, Saccharomyces cerevisiae, is a purely respiratory process. Endogenous respiration is KCN-labile. Our own experiments together with various accounts and data given in the literature show that the same "oxygen-transporting mechanism" functions in both endogenous and exogenous metabolism. However, the lack of sensitivity of the endogenous system of reactions to low concentrations of monoiodoacetic acid, the absence of anaerobic CO(2) production, and the absence of alcohol production, demonstrate that fermentation is not involved in the dissimilation of the carbohydrate reserves. Throughout the experiments the endogenous respiration behaved functionally as a unitary system of reactions. The O(2) consumption and CO(2) production were parallel at all times; i.e., the R. Q. was consistently 1. Monoiodoacetic acid and KCN in concentrations from 10(-5) to 10(-1) molar affected both O(2) uptake and CO(2) production to the same extent. The only agents known to alter the value of the R. Q. were those which disrupted the normal protoplasmic structure, viz. grinding the cells with sand, plasmolyzing them with toluol and hypertonic salt solutions, or pressing them in a hydraulic press. These agents brought about a vigorous anaerobic CO(2) production accompanied by an accumulation of alcohol in the medium. The unitary character of endogenous respiration is exhibited only when the normal structure of the cell is kept intact; apparently it depends upon the maintenance of a chambered (or compartmental) architecture of the cell.