quantities of material without being destroyed, their mere presence under suitable conditions being all powerful to produce profound alterations, these enzymes play a peculiar part. Present in mere traces, they are able to transform many thousand times their weight of proteid matter into soluble and diffusible products. All that is essential is their mere presence under suitable conditions, and strangely enough the causative agent itself appears to suffer no marked change from the reactions set up between the other substances.

There are many theories extant to explain this peculiar method of chemical change, but few of them help us to any real understanding of the matter. These enzymes are typical catalytic or contact agents, and by their presence render possible marked changes in the character of the proteid or albuminoid matter with which they happen to be in contact. But the conditions under which the contact takes place exercise an important control over the activity of the ferment. Temperature, reaction, concentration of the fluid, presence or absence of various foreign substances, etc., all play a very important part in regulating and controlling the activity of these two proteolytic enzymes. In fact, as one looks over the large number of data which have gradually accumulated bearing upon this point, one is impressed with the great sensitiveness of these ferments toward even so-called indifferent substances. Their specific activity appears to hinge primarily upon the existence of a certain special environment, alterations of which may be attended with an utter loss of proteolytic power, or, in some less common cases, with a decided increase in the rate of digestive action. This constitutes one of the peculiar features of these proteolytic enzymes; powerful to produce great changes, they are nevertheless subject to the influence of their surroundings in a way which testifies to their utter lack of stability. Furthermore, as you well know, conditions favorable for the action of the one ferment are absolutely unfavorable for the activity of the other, and indeed may even lead to its destruction. Thus, while pepsin requires for its activity the presence of an acid, as 0.2 per cent. HCl, trypsin is completely destroyed in such a medium. Again, trypsin exhibits its peculiar proteolytic power in the presence of sodium carbonate, a salt which has an immediate destructive action upon pepsin. Hence, a medium which is favorable for the action of the one ferment may be directly antagonistic to the action of the other.

Another factor of great moment in determining the activity of these two enzymes is temperature. That which is most favorable for their action is 38° to 40° C., and any marked deviation from this temperature is attended by an immediate effect upon the proteolysis. Exposure to a low temperature simply retards proteolytic action, doubtless in the same manner that cold checks or retards other chemical changes. There is no destruction of the ferment, even on exposure to extreme cold, the enzyme being simply inactive for the time being. Exposure of either pepsin or trypsin to a high temperature, say 80° C., is quickly followed by a complete loss of proteolytic power, i.e., the ferment is destroyed. It is to be noticed, however, that the destructive action of heat is greatly modified by the attendant circumstances. Thus, fairly pure trypsin, dissolved in 0.3 per cent. sodium carbonate, is completely destroyed on exposure to a temperature of 50° C. for five to six minutes, while a neutral or slightly acid solution of the pure enzyme is destroyed in five minutes by exposure to a temperature of 45° C. On the other hand, the presence of inorganic salts and the products of digestion, such as albumoses, amphopeptone, and antipeptone, all tend to protect the trypsin somewhat from the destructive effects of high temperatures, so that in their presence the enzyme may be warmed to 60° C. before it shows any diminution in proteolytic power. Alkaline reaction, combined with the presence of salts and proteid, viz., just the conditions existent in the natural pancreatic secretion, constitute the best safeguard against the destructive action of heat, and under such conditions trypsin may be warmed to about 60° C. before it begins to suffer harm. But all this testifies in no uncertain way to the extreme sensitiveness of the ferment to changes in temperature; a sensitiveness which manifests itself not only in diminished or retarded proteolytic action, but terminates in destruction of the ferment when the temperature rises beyond a certain point.

Similarly, pepsin dissolved in 0.2 per cent. hydrochloric acid feels the destructive effect of heat when a temperature of 60° C. is reached. In a neutral solution, on the other hand, destruction of the ferment may be complete at 55° C. Here, too, peptone retards very noticeably the destructive action of heat, especially in an acid solution of pepsin, so that under such circumstances the ferment may not be affected until the temperature reaches 70° C. I have tried many experiments along this line, not only with pepsin and trypsin, but also with many other ferments. We may briefly summarize, however, all that is necessary for us to consider here in the statement that the pure isolated ferments are far more sensitive to the destructive action of heat than when they are present in their natural secretions. This, as stated, is due not only to the reaction of the respective fluids but also to the protective or inhibitory action of the inorganic salts and various proteids naturally present. We may thus say with Biernacki36 that the purer the ferment the less resistant it is to the effects of heat.

It is thus plain that these enzymes, capable though they are of accomplishing great tasks, are nevertheless exceedingly unstable and prone to lose their proteolytic power under the slightest provocation. When, however, they are surrounded by their natural environment, the acid or alkali of the respective secretion, together with salts and proteids, they then appear more stable; their natural lability becomes for the time being transformed into semi-stability, and the temperature, for example, at which they lose their peculiar power, is raised ten degrees or more. I have also found the same to be true of the vegetable proteolytic ferments, and also of the amylolytic ferment of saliva.

The above facts furnish us, I think, a good illustration of how dependent these proteolytic enzymes are upon the proper conditions of temperature, to say nothing of other conditions, for the full exercise of their peculiar power. Toward acids, alkalies, metallic salts, and many other compounds they are even more sensitive than toward heat, and much might be said regarding the effects, inhibitory or otherwise, produced by a large number of common drugs or medicinal agents on these two ferments. Any lengthy discussion of this matter, however, would be foreign to our subject, and I will only call your attention in passing to one or two points which have a special bearing upon the general nature of the enzymes. Take, for example, the influence of such substances as urethan, paraldehyde, and thallin sulphate on the proteolytic action of pepsin-hydrochloric acid37 and we find that small quantities, 0.1 to 0.3 per cent. tend to increase the rate of proteolysis, while larger amounts, say one per cent., decidedly check proteolysis.