The Requirement of Oxygen for the Active Transport of Sugars into Yeasts

Abstract

The inducible systems of uptake of β-d-galactopyranosides into Kluyveromyces fragilis, Kluyveromyces marxianus and Debaryomyces polymorphus were studied under aerobic and anaerobic conditions, using both lactose and the non-metabolized analogue, methyl 1-thio-β-d-galactopyranoside (TMG). A common carrier served both substrates, and aerobic entry was by active transport, involving proton symport. The rate of uptake and the final equilibrium concentration were decreased on adding N,N′-dicyclohexylcarbodiimide, diethylstilbestrol or antimycin A, and active transport was completely abolished by carbonyl cyanide m-chlorophenylhydrazone. Uptake under anaerobic conditions differed markedly from that occurring aerobically: TMG was not concentrated anaerobically, even by strongly fermenting yeasts. Instead, it was transported into the cells by facilitated diffusion, which could sustain a rate of entry of over half the maximum rate observed aerobically. That this difference between aerobic and anaerobic transport into yeasts might apply to glycosides in general, was suggested by the finding that a strain of Saccharomyces cerevisiae also took up maltose by active transport under aerobic conditions, but by facilitated diffusion anaerobically. By contrast, the amino acid, 2-aminoisobutyric acid, was concentrated by Kluyveromyces fragilis, even under anaerobic conditions. The significance of these findings in relation to the Kluyver effect is discussed.