Surface activity as the cause of the anomalous behavior of urethane as an inhibitor of yeast respiration.

The oxidation of glucose by yeast is inhibited by urethane, and at sufficiently high concentrations of the inhibitor oxygen uptake may be suppresed completely. However, no direct proportionality exists between the degree of inhibition and urethane concentration. This discontinuity in the relationship between degree of inhibition and concentration of urethane has been adduced by Fisher and Stern (1942) as evidence for the existence of two parallel and independent systems of respiration differing in their sensitivity to urethane. The more sensitive system has been labeled by these investigators as an "activity" respiration specifically providing the energy utilized in assimilation for growth and multiplication. It does not seem to be realized generally that urethane is a nonionic surface active agent. The surface activity of urethane is an expression of its 0 11 composition, NH2C-OCH2CH3, which includes polar structures at one end of the molecule and the nonpolar ethyl group at the other end. Within the range of concentrations generally employed in metabolic studies the surface tensions of aqueous solutions of urethane at 28 C fall from a value of 66.5 for a 0.1 M solution to 51 dynes per cm2 at 1.0 M. At the same temperature the surface tension of distilled water is approximately 71 dynes per cm2. In evaluating the influence of concentration on the toxicity of a metabolic inhibitor it must be recalled that the suspension of organisms in a solution of the inhibitor acts as a system of discontinuous phases. The effective concentration of the inhibitor is not the concentration added to the system but is the concentration of the drug accumulating in the interface at the bacterial surface. Characteristically the concentration of a surface active agent will tend to be greater at the interface between the solution and the bacterium than it will be in the bulk of the solvent phase. It is an important fact, too, that under a fixed set of conditions a change in concentration of a surface active agent is not accompanied by a proportionate change in the concentration of the surface active agent at an interface nor, therefore, of interfacial tension. The Freundlich and Langmuir adsorption isotherms are mathematical expressions of this finding. Since micelle formation tends to occur only within certain ranges of concentration of the surface active agent, discontinuities observed in interfacial tension-surface active agent concentration curves also may be due to micelle formation or the aggregation of solute molecules. That this latter factor is an important cause of abrupt change in the slope of surface tensionsolute concentration curves has been shown to be true for even so simple a surface active substance as butyric acid (Grindley and Bury, 1929). Similarly other physical characteristics of butyric acid, such as density, freezing point, electric conductivities, and vapor pressure, show a discontinuous relationship when the values are plotted against concentration. Biologically this phenomenon is important since organisms may not respond in an identical manner to individual molecules and aggregates of the same substance. For one thing the permeability of biological membranes to aggregates would probably be less than to individual molecules. In the present report data will be presented which indicate that the discontinuous curves obtained when oxygen consumption of yeast is plotted against urethane concentration are similar to the curves obtained by plotting the surface tension of the solutions against urethane concentration. In the light of the principles outlined above this finding suggests that the discontinuities in the relation of urethane concentration to oxygen consumption are a reflection of the surface active nature of urethane and not of the existence of independent mechanisms of respiration of different degrees of sensitivity to urethane. Such a conclusion is fortified by the demonstration in