An outer metabolic region of the yeast cell.

During fermentation with baker's yeast, free suecinic acid appears rapidly in the suspending medium (Conway & Brady, 1947). With 1 part of washed centrifuged yeast to 0-6 part of 5% (w/v) glucose, forming a mixture with approximately 0-8 part of cells to 0-8 part of external fluid, about 40 m-equiv. of succinic acid per kg. cells diffuse out within 30 min., the pH of the suspending fluid falling to about 3. If the succinic acid diffused throughout the yeast cell, then under such conditions the total succinic acid plus succinate within the cells should be much higher than outside; but it is found to be about one-tenth of the external value. When resting cells are suspended in 0-2M-succinic acid the same relation holds. For the resting cells the ratio of the inner to outer concentrations using 0-2M-acid is only a little increased beyond the level in fermentation, and it does not alter significantly over many hours. Such succinic acid, though not diffusing appreciably into the whole cell, yet enters some real space therein. Inulin does not enter this region, neither do peptone, used by Montgomery & White (1945) to determine the intercellular space, nor gelatin. The bivalent ions of succinic acid appear debarred, and the space can be regarded as having inner and outer membranes with different permeabilities. The view that free succinic acid, which must be formed in the cell, is prevented from diffusing backwards by some energetic process without a membrane barrier, may be shown to be untenable from the rapidity of the diffusion under such circumstances and the magnitude of the required energy. This is in no way opposed to the view that the succinate ion may be carried from within the cell outwards across the inner membrane of the outer region, by a process involving the expenditure of energy, combined with a special redox carrier. Evidence for such a process is being presented in a subsequent communication. Such data related to succinic acid diffusion, and many other facts ofa similar kind, are here presented as showing the existence of an outer metabolic region of the yeast cell (Conway & Downey, 1948), and for an approximate determination ofits volume; also for the view that the region may be identified with the cell wall. In relation to this outer region, and in a preliminary way, the permeability of the cell to some groups of substances of physiological importance have been examined, this study being related to that on acid production described in subsequent papers.