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

Abstract

IIn a previous paper concerned with a kinetic analysis of endogenous respiration of bakers' yeast (Stier and Stannard, 1935-36) it was shown that the rate of respiration was a function of the time in the "non-nutrient" medium, the age of the cells, and their previous history. The rates of both O, consumption and CO, production were equal under almost all conditions. Evidence was summarized to show that the dissimilated substrate is a carbohydrate, probably glycogen. The statement was made that the term endogenous respiration is preferable to "autofermentation" in referring to the metabolic utilization of stored glycogen in intact bakers' yeast cells. This change in terminology was based mainly upon the results of investigations reported in the present paper.It is usually assumed that the decomposition of stored glycogen proceeds within the yeast cell by the same mechanisms as the respiration and fermentation of simple carbohydrates placed in the external medium.l We will show in this account that dissimilation of the available carbohydrate reserves proceeds by way of a "unit" chain of reactions resulting in a complete oxidation of the carbohydrate to CO, and I-I,O. Since "pure" respiration only is involved we may speak of "the respiratory character of endogenous metabolism" or may refer directly to the process as "endogenous respiration." We will also show that the endogenous chain of reactions behaves as a distinct functional unit only when the structural integrity of the cell is maintained. Its unitary character is thus the property of a "chambered architecture" of the yeast cell (cf. Hofmeister, 1901).1 By fermentation we mean anaerobic CO, production and the accumulation of alcohol in the medium.

Role of Electrolytes and Starvation in Altering Apparent Radiosensitivity of Bakers' Yeast

“…The effect of starvation is undoubtedly not solely due to loss of cations. Depletion of enzyme and/or substrate reserves is known to occur (Stier and Stannard (21)) and this may account for the failure of added potassium to restore the fermentation completely. However, we have not been able to increase the radiosensitivity of our yeast by growing it in a nltrogen-deficient medium (2).…”

“…To explore these effects further, two procedures were adopted: (a) starving the yeast by 24 hours' aeration in distilled water and (b) exposure of the yeast to the cation exchange resin, dowex 50, in order to deplete the intracelluiar cation concentration. Starving has been shown to lower the intraceUular potassium level from 0.1 M to 0.07 ~ (12) and also to decrease substrate reserves (21), the presence of which may provide a protective action by competing for the ionization-produced radicals.…”

Studies on the energy metabolism during anaerobic fermentation of glucose by baker's yeast

“…As m a y be seen from Table I W h e n glueose was eliminated from the m e d i u m a considerable endogenous heat development was observed. This was rather u n e x p e e t e d sinee m a n y referenees in the literature [4,8] When the values for heat production in the presence of glucose are corrected by substracting this endogenous heat development one obtains values for heat development per mm a CO 2 produced that are within the expected limits of 670 to i220 ~cal/mm 8 CO~ produced; however, such an extensive correction is highly unsatisfaetory.…”

On the Relation Between Respiration and Fermentation in Yeast and the Higher Plants

“…In a few cases it has recently been demonstrated that some cells may live for many hours in nitrogen unharmed while producing no carbon dioxide at all. Thus Steier & Stannard (1936) show that although yeast cells when washed completely free of the sugar solution in which they were grown can carry out endogenous respiration at a very low rate, they do not, when undamaged, give out any appreciable quantity of carbon dioxide in nitrogen. The same has been demonstrated for some aerobic bacteria and for Chlorella cells under certain conditions.…”