The direct fermentation of maltose by yeast. 2

Abstract

Expts. are reported dealing with the relative role and nature of the direct and indirect enzyme mechanisms of maltose fermentation in yeast.[long dash]A. Baker''s yeast. Different agents may selectively inhibit or stimulate maltose fermentation by baker''s yeast without similar effect on glucolysis or maltase activity. Maltose fermentation was stimulated by any of the zymohexoses but not by galactose. Methyl a-glucoside is a powerful substrate-specific retardant of maltose fer-mentation by baker''s yeast. The inhibition by methyl a-glucoside is completely reversible. Its intensity increases with increasing conc. of glucoside. Methyl /3-glucoside and methanol in equivalent concentration did not inhibit mal-tose fermentation. High concentrations of maltose may inhibit the rate at which maltose is fermented by aged baker''s yeast but do not similarly affect glucose fermentation or maltase activity in vitro. Addition of glucose traces abolishes the retardant effect of high maltose conc. The pH-activity curve of cell-bound maltase is nearly identical with that of cell-free maltase, distinct from that of glucozymase, and actually opposite in trend to the pH-activity curve of maltose fermentation. Maltose is vigorously fermented by living baker''s yeast at acid pH values which preclude hydrolysis by either cell-free or cell-bound maltase, but was not fermented at all by the same cells at neutral pH, which is optimal for maltase. The conclusion follows that maltose fermentation by baker''s yeast is direct. The view is advanced that the maltozymase complex of yeast involves specific maltose fermentation catalysts and that these are generally formed in the cell during maltose fermentation. Methyl a-glucoside only retarded maltolysis when added to the fermentation mixture before maltose fermentation had become vigorous. Traces of glucose markedly curtailed, but never entirely abolished, the induction period preceding the onset of the stationary phase of maltose fermentation. Baker''s yeast cells failed to ferment maltose at neutral pH, and methyl a-glucoside at any pH, but acquired ability to ferment these substrates at pH 7.0 when dried. The fresh cells yielded active typical maltase on autolysis. It seems necessary to conclude that these yeast cells may contain maltase in inactive forms (maltase zymogens).[long dash]B. Brewer''s yeast. Maltose fermentation by brewer''s yeast differed in several respects from that by baker''s yeast. Ordinarily samples of brewer''s yeast fermented maltose and glucose at an equal rate. As expected, maltose fermentation was here found insusceptible of inhibition by methyl a-gluco.side or of activation by traces of hexose. At high substrate conc., maltose was in certain cases fermented more rapidly than glucose. Maltose fermentation by brewer''s yeast occurs at neutral as well as at acid pH (3-7). The same cells ferment methyl [alpha]-glucoside. vigorously in the neutral range, but not at all below p.H 4. They may be concluded to contain active maltase. It is suggested that at acid reaction, brewer''s yeast ferments maltose by means of a direct fermentation mechanism (maltozymase) and that an indirect fermentation mechanism-maltase glucozymase is solely or largely responsible for fermentation of maltose in the neutral range.