Proteolytic Enzymes and the Question of Protein Storage. ∗

Abstract

Many observers have noticed that the nitrogen excretion of an animal on a N-free diet will decline for long periods of time. This fact is difficult to explain on the basis of the depletion of a supply of “reserve” or “deposit” protein. The amounts of nitrogen excreted may account for a much larger storage of protein in the body than anyone has ever been able to demonstrate. That the decline in the rate of excretion of nitrogen may be attributed to a change in the mechanism for the catabolism of protein was suggested by the author several years ago. 1 The assumption is made that an animal with an enzyme system elaborated to catabolize excess dietary protein, which may very likely first be converted to tissue protein, will go on tearing down tissue protein when the supply of dietary protein is suddenly removed. Gradually, however, as the animal continues to live on a diet free of protein, the enzyme system will deteriorate and the catabolism of protein will decrease.

This contention is borne out by the data of Table I. These data show the arginase content of the livers of rats after living on a N-free diet more than 30 days, as compared with normal controls taken from a stock diet.

The method for arginase assay was a modification of that described by Lightbody. 2 A 30-minute digestion period at 374dG and pH 9.4 was followed by aeration of the ammonia liberated at pH 5.0 by glycerol urease. Manganous sulfate was used as an activator. Every precaution was taken to treat identically the samples taken from the experimental and control livers. Of course the N-free animals lost considerable body weight while subsisting on the N-free diet.