Oxidative pathways of ethanol metabolism in baker's yeast

Abstract

Ethanol oxidation by baker’s yeast has been studied by respirometric determinations. It was shown that within certain limits of ethanol concentrations (4.10^-4 M to 2,7.10^-3 M), the rate of oxygen uptake, by a suspension of yeast cells (A = 1.0 at 460 nm) is proportional to the ethanol concentration. Only for 4.1.10^-4 Methanol, oxygen uptake reached the calculated theoretical rate. When suspensions of cells ranging from 0,4 to 1,0 of absorbance were incubated with 1.3.10^-3 M ethanol, the QO₂ values calculated as oxygen uptake per mg protein were about the same at 20 min. of incubation but steadily decreased as the number of cells increased when the incubation time was longer. Respiratory inhibitors as veronal and sodium azide strongly inhibit oxygen uptake. Alkylating compounds as iodoacetamide act as much better inhibitors than the mercaptide forming agent p-hydroxymercurybenzoate. Malonate ( 10^-2 M) inhibits oxygen uptake only in 4.5. 40 per cent of the total, oxygen uptake measured in systems containing KOH as CO₂ absorvent were found to be consumed in the absence of KOH, a fact that can be explained in terms of the direct oxidation by the yeast mitochondria of the NADH + H + originated from the oxidation of ethanol by alcohol dehydrogenase; 60 per cent of the oxygen uptake can be accounted by the catabolic fate of acetil-S-Co A originated from the ethanol oxidation.