Crystalline Acetyl Derivatives of Pepsin

Abstract

The isolation and identification of several individual enzymes has made it possible to attack the problem of their chemical structure. In the last few years it has been shown that urease (1), pepsin (2), trypsin (3), chymo-trypsin (4), probably pancreatic amylase (5) and rennin (24) are proteins, so that the general molecular structure of these enzymes is known. Enzymes have powerful catalytic properties, however, not possessed by ordinary proteins and therefore must possess characteristic chemical structures.Since the chemical properties of a molecule are not the sum of the properties of its component parts, the catalytic activity of an enzyme must be considered as a property of the whole molecule and cannot, strictly, be referred to one special group. It is frequently true, however, that a particular property of the molecule is dependent upon a particular group or arrangement of the atoms within the molecule; the magnitude of this property depends upon the structure of the whole molecule. Thus organic acids usually contain a carboxyl group but the strength of the group depends on the entire molecular configuration. In the case of proteins possessing peculiar properties the first question to arise is whether the molecule consists entirely of amino acids or whether it contains a prosthetic group; i.e., a compound not composed of amino acids. The peculiar properties of hemoglobin are ascribed to the presence of the prosthetic group, heine, which can easily be separated from the protein, globin. In this case the prosthetic group may be detected by its color. The yellow respiratory ferment of Warburg (6) is similar since again the prosthetic group is colored. In both these cases the characteristic properties are lost completely when the prosthetic group is separated from the rest of the molecule. The prosthetic group alone is therefore not responsible for the activity.

Inactivation of Pepsin by Iodine and the Isolation of Diiodo-Tyrosine From Iodinated Pepsin

“…Protein Nitrogen.--This estimation was made as the difference between the total nitrogen and the non-protein nitrogen, the details being described in a previous paper (1).…”

“…The previous work of the writer on the aeetylafion of pepsin (1,2) points to a close relationship between the tyrosine of pepsin and the proteolytic activity of the protein molecule. An acetyl derivative of pepsin was prepared which had 60 per cent of the activity of pepsin, contained three labile acetyl groups, and gave a tyrosine-phenol color value lower by three tyrosine equivalents than that of pepsin.…”

Note on the action of acetylating agents on amino-acids

“…Action of ketene on amino-acids and related compounds Ketene was generated from acetone as described by Herriott [1934] in an all-glass apparatus in order to avoid the formation of polymerization products.…”