In these samples of yeast-juice, therefore, there is present a natural condition of affairs precisely similar to that obtaining in the artificial mixtures of inactive filtration residue and co-enzyme solution made by Harden and Young. The balance of quantities is such that the co-enzyme disappears before the enzyme, leaving a certain amount of enzyme capable of exercising its usual function as soon as sufficient co-enzyme is added. This establishes an interesting point of contrast with the juice prepared from top yeast in England, in which the enzyme does not outlast the co-enzyme [Harden and Young, 1907]. The difference may be due to some variation in the relative proportions of enzyme and co-enzyme or of the enzymes to which the disappearance of each of these is presumptively due, or to a combination of these two causes. It was, however, found, even in the juice from bottom yeast, that incubation for three days at 22° without the addition of sugar caused the disappearance of the enzyme as well as of the co-enzyme, and left a residue alike incapable of being regenerated by the addition of co-enzyme or of restoring the power of producing fermentation to an inactive mixture containing enzyme and sugar. If the fermenting power of the juice is to be preserved by repeated regeneration for a long period, it is absolutely necessary to add the co-enzyme solution each time as soon as fermentation has ceased, since the enzyme in the absence of this addition rapidly disappears, even in the presence of sugar. This result is probably to be explained, at all events in the main, by the presence in the co-enzyme solution of the antiprotease to which reference has already been made [Buchner and Haehn, 1910, 2]. This agent, the constitution of which is still unknown, protects proteins in general from the action of digestive enzymes, and on the assumption that the alcoholic enzyme of yeast-juice belongs to the class of proteins, may be supposed to lessen the rate at which this enzyme is destroyed by the endotryptase of the juice. This antiprotease is, like the co-enzyme (p. 68), destroyed by lipase but is more stable than the co-enzyme towards hydrolytic agents, and can be obtained free from co-enzyme by boiling the solution for some hours alone or by heating with dilute sulphuric acid. Such a solution possesses no regenerative power, but still retains its power of protecting proteins against digestion and of preserving the fermenting power of yeast-juice. [p066] It must, however, be remembered that the addition of a phosphate alone may greatly prolong the period of fermentation of yeast-juice (p. 55), and sugar is well known to exert a similar action. It appears, therefore, that the existence of the enzyme is prolonged not only by the presence of the antiprotease but also by that of sugar and hexosephosphate, into which phosphate passes in presence of sugar. Similar effects are exerted on the co-enzyme by sugar and probably also by hexosephosphate.