Effect of 4-6 Dinitro-O-Cresol on Oxidation of d'- and l'-Arabinose by Previously Starved Yeast. ∗

Abstract

It has been shown in previous papers from, this laboratory that the respiratory rate of yeast suspended in non-nutrient phosphate buffer solutions or in glucose phosphate can be markedly increased on treatment with proper concentrations of 2–4 dinitrophenol (DNP), 4–6 dinitro-o-cresol (DOC) and some related compounds. 1 – 5 We report herewith the results of experiments in which it is shown that the effects of such metabolic stimulants upon the respiration of yeast is profoundly affected by the type of “exogenous” 6 carbohydrate fuel available.

A pure culture of Saccharomyces cerevisiae and pure culture methods were used throughout. The experimental procedures were the same as described previously 2 , 3 except that yeast was first grown 48 hours on Orla-Jensen agar medium, then taken up in 0.2M phosphate buffer, pH 6.8, and starved 24 hours under aerobic conditions in Fraser tubes 7 at 25° ± 0.02°C. before the beginning of each experiment. Respiration was measured in the conventional form of Warburg apparatus at 25° ± 0.02°C. Each set of quantitative comparisons was made on yeast from a single subculture. The observations reported represent typical findings in a series of 8 experiments involving 7 Warburg vessels each.

Fig. 1 illustrates integral curves showing the total amount of oxygen consumed in cmm. at N.P.T. as a function of time when previously starved yeast is suspended in 0.1M phosphate buffer, pH 6.8, or in the same buffer containing 1% glucose (for comparison), 1% d'-arabinose (levo-rotatory) or 1% l'-arabinose (dextro-rotatory).

In this experiment DOC was added from the sidearms of the Warburg vessels after a 60-minute control period. The strength of the DOC was such as to furnish a concentration in the Warburg vessel of 5.05 × 10⁻⁴M (giving 7.96 × 10⁻⁶M free acid at pH 6.8), which was found optimal in a series of preliminary experiments. After addition of DOC, the slope of the integral curves in the presence of glucose (not shown in Fig. 1, but can be inferred from Fig. 2) and of 1'-arabinose is greater, in the presence of d'-arabinose the same, as in the non-nutrient control. Since the integral curves for the two pentoses practically coincide before addition of DOC and draw apart afterward (Fig. l), this finding constitutes an interesting case of stereoisomeric preference on the part of the yeast cell.