One of the most interesting and at the same time most difficult problems concerning enzyme action in general is the nature of the inhibiting or accelerating effect produced by many substances upon the rate or total result of the chemical process set up in presence of the enzyme. Inhibition, it is usually supposed, involves either the decomposition of the enzyme, in which case it is irreversible, its removal from the sphere of action by some change in its mode of solution, or the formation of an inactive or less active compound between the enzyme and the inhibiting agent. This compound it may sometimes be possible to decompose, with the result that the activity of the enzyme is restored. A striking example of this, to which allusion has already been made, is the effect of hydrocyanic acid on alcoholic fermentation (p. 37). Acceleration of enzyme action can in some cases be ascribed to the fact that the accelerating substance possesses an assignable chemical function in the reaction, so that an increase in the concentration of this substance causes an increase in the rate of the reaction. As we have seen in Chapter III, this is the explanation of the accelerating effect of phosphates on fermentation by yeast-juice. In many other cases, however, no such chemical function can be traced, as, for example, in the effect of neutral salts on the hydrolytic action of invertase, or the effect of the addition of the co-enzyme to zymase, and it is necessary to fall back on some assumption, such as that the accelerating agent acts by increasing the effective concentration of the enzyme or by combining either with the enzyme or the substrate, forming a compound which undergoes the reaction more readily. The interest in the following examples of inhibition and acceleration of fermentation by yeast-juice lies not only in their relation to these general problems but also, and perhaps chiefly, in their bearing on the specific problem of the nature and mode of action of the various agents concerned in the production of alcohol and carbon dioxide from sugar in the yeast-cell. [p071] I. INFLUENCE OF CONCENTRATION OF PHOSPHATE ON THE COURSE OF FERMENTATION. Prominent among these instances of inhibition and acceleration are the phenomena attendant on the addition of excess of phosphate to yeast-juice. When a phosphate is added to a fermenting mixture of a sugar and yeast-juice, the effect varies with the concentration of the phosphate and the sugar and with the particular specimen of yeast-juice employed. With low concentrations of phosphate in presence of excess of glucose the acceleration produced is so transient that no accurate measurements of rate can be made. As soon as the amount of phosphate added is sufficiently large, it is found that the rate of evolution of carbon dioxide very rapidly increases from five to ten times, and then quickly falls approximately to its original value.