Transport of sugars in yeasts: II. Mechanisms of utilization of disaccharides and related glycosides

Abstract

1. 1. The outstanding ability of some yeasts for the fermentation of certain disaccharides and the recent identification of specific transport systems for the uptake of hexoses prompted the investigation of the likely involvement of transport processes in the fermentation of oligosaccharides. 2. 2. The kinetics and specificity of the fermentation of the main types of glycosides by intact yeasts and of their hydrolysis by cell-free extracts have been studied. 3. 3. Extracellular trapping of hexoses can interfere with the fermentation of β-fructosides by S. cerevisiae and with that of α-galactosides by S. carlsbergensis. This observation added to the fact that the pH-activity curve and substrate specificity are the same for fermentation and hydrolysis, demonstrates that the first step in the fermentation of sucrose, melibiose, and their analogues is a hydrolysis outside the membrane, followed by transport of the liberated hexoses. 4. 4. On the contrary, the utilization of maltose by S. cerevisiae and that of lactose by S. fragilis show substrate specificity and pH-activity curves markedly different in vivo and in vitro. Maltose and lactose fermentation by intact yeasts can be faster than that of their constituent hexoses and was not affected by the addition of a system for the trapping of extracellular hexoses. The combined evidence strongly suggests that specific transport systems are the first step in the fermentation of maltose and lactose, followed by splitting by intracellular enzymes.