The Action of Nitrophenols on Respiration and on Glucose Assimilation in Yeast

Abstract

The consumption of 2·25 mg. glucose by a resting suspension of baker's yeast in the presence of finely graded concentrations of 3:5 -dinitro-o-cresol was followed at pH 2·4 and 5·0 by measuring oxygen uptake and carbon-dioxide output; at pH 8·5 oxygen uptake alone was measured. At each pH certain concentrations of dinitrocresol stimulated respiration and increased the ratio of glucose oxidized to glucose assimilated; at higher concentrations, assimilation was reduced to zero, but respiration became inhibited and the whole of the glucose was then metabolized by aerobic fermentation. Dinitrocresol is believed to be acting in these experiments as an uncoupling agent, lowering the net rate of formation of energy-rich phosphate and so allowing respiration to proceed faster while assimilation, an endergonic process, is inhibited. The maximum respiratory stimulation obtainable at pH 8·5, 325 per cent, of the control, is greater than can be obtained at pH 2·6 or 5·0, which may be due to the slower respiration rate of controls at pH 8's. The slow endogenous respiration can also be stimulated more than the exogenous. With o - and p -nitrophenols similar relations are obtained, but phenol does not stimulate respiration, although it inhibits assimilation in lower concentrations than respiration. Under nitrogen, a stimulation of fermentation can be obtained with dinitrocresol at pH 5·0 but not at 2·6. At both pH levels, assimilation is more easily suppressed than fermentation rate.