In 1902 Pekelharing prepared what has generally been regarded as probably the purest pepsin of which we have record. This product contained carbon, hydrogen, nitrogen, and sulphur in proportions within the range of variation found among ordinary proteins.† It also behaved like ordinary proteins in the xanthoproteic test and Millon reaction and in showing the presence of the tryptophane group.

1 W. Beaumont. Experiments and Observations on the Gastric Juice and the Physiology of Digestion. Plattsburg, 1833.

2 Eberle. Physiologie der Verdauung nach Versuchen. Wurzburg, 1834.

3 Schwann. Ueber das Wesen der Verdauungs processe. Muller's Archiv, 1836, pages 90-138.

*Those students not yet familiar with the names of the common enzymes should perhaps read first the sections on classification and terminology below.

† A small amount of chlorine shown by Pekelharing's preparation was later found by Dezani to be not an. essential constituent but probably due to incomplete removal of the hydrochloric acid with which pepsin is associated in the gastric juice.

Dezani, in 1910, carried forward the work upon the chemical nature of pepsin by preparing what was believed to be a substantial duplicate of Pekelharing's product and submitting this to hydrolysis, followed by search for individual hydrolytic products according to the methods which had recently been developed in the study of the structure of the proteins. He demonstrated the presence of leucine, tyrosine, arginine, histidine, and lysine and also found evidence of other amino acids which the limitations of his material and methods did not permit him to identify.

Thus pepsin as prepared by Pekelharing and by Dezani is a nitrogenous material not identical with any other known substance but complying with the criteria of our present conception of a protein in elementary composition, in color reactions, and especially in yielding the familiar amino acids upon hydrolysis. Recent studies by Aldrich also indicate that the chemical nature of pepsin is that of a protein.

It must be borne in mind that the criteria of purity usually applied in chemical investigations are not applicable to enzyme preparations because of their colloidal nature and the readiness with which their characteristic properties are destroyed. Yet in view of the fact that, with very few if any exceptions, the changes by which the organic foodstuffs are prepared for absorption in the digestive tract and are utilized in the body tissues are dependent upon the presence of enzymes the material for whose synthesis must in the long run be furnished by food, we should not be deterred by the inherent difficulties and uncertainties of the subject from the study of such evidence regarding the chemical nature of the enzymes as can be obtained; nor are we at present quite so much in the dark as the statements in most textbooks would seem to indicate.

Several years earlier than Pekelharing's work on pepsin, Osborne 1 had published an investigation of the chemical nature of diastase (malt amylase), which may be regarded as marking the beginning of our modern knowledge in this field. From this work it appeared that the enzymic activity is a property of a definite fraction of the protein material of the malt, or in other words that the enzyme is protein in its chemical nature. Although criticized by some, Osborne's findings have been confirmed by the most recent investigations. Since space permits here only the discussion of those enzymes which are directly concerned in digestion, the reader must be referred to the original papers for an account of Osborne's methods and results.

1 T. B. Osborne. Journal of the American Chemical Society, Vol. 17, page 587 (1895); Vol. 18, page 536 (1896).

Of the two amylases concerned in the digestive process, ptyalin of saliva and amylopsin of the pancreatic juice, only the pancreatic amylase has been studied by modern methods with reference to its chemical nature.

In an investigation 1 in which the attempts at purification were guided and their success largely judged by quantitative determinations of the starch-digesting action of the products there was developed a method of purification which in numerous independent experiments yielded a product that was not only extraordinarily active in the hydrolysis of starch but was essentially uniform both in digestive activity and in chemical nature. This result strongly suggests that the product was not merely an indefinite mixture but represented at least some approximation toward an actual isolation of the enzyme. These preparations show the composition and color reactions of typical proteins and, like Osborne's malt amylase, the material when heated in water solution yields an albumin coagulum and a proteose or peptone which remains in solution. Moreover, on hydrolysis the material yields the same groups of amino acids which are yielded by typical proteins such as casein, which it also resembles in elementary composition.

While the chemical nature of the lipases of the digestive tract has not been studied, Falk and Sugiura have shown that the purified lipase preparations made from castor beans are, like the proteases and amylases above mentioned, essentially protein material.*

1 Journal of the American Chemical Society, Vol. 33, page 1195; Vol. 34, page 1104; Vol. 35, page 1790.

The materials obtained in attempts to isolate enzymes are here called merely products or preparations; it is not stated that any enzyme has been perfectly separated and purified. As already explained, the familiar criteria of purity are not applicable to these unstable colloidal substances. It is possible that the enzymes in the purified preparations mentioned above may still be mixed with considerable amounts of other substances, and it has even been suggested that the protein material of which the above-mentioned enzyme preparations are chiefly composed may be present only as a carrier and that the actual enzyme may be a substance of a different nature. There is, however, no direct evidence in favor of this suggestion. The facts now available make it altogether probable that the typical enzymes concerned in the utilization of the foodstuffs either are modified proteins or contain protein as an essential component. In this case the food protein must furnish material for body enzymes as well as for body tissue.