The Metabolism of Proteins and Amino Acids

Abstract

Perhaps the most significant series of investigations on the metab olism of proteins and amino acids, extending into the current year, has been carried out in this country by a group of scientists headed by Rudolf Schoenheimer, whose presence in this country was more a matter of circumstance than of choice. If political intolerance can be said to have any redeeming features, it would be in the enrichment of the intellectual life of those countries not so afflicted, by the en forced migration of scholars of distinction in their chosen fields of study. Among these scholars, Schoenheimer stood pre-eminent. His tragic death on September 11, 1941, must be recorded with sincere regret.The investigations of Schoenheimer and colleagues, involving the use of amino acids and other nitrogenous compounds that had been labeled by the inclusion in their molecules at strategic points of the N15 isotope and of deuterium, permitted a direct attack upon many problems of intermediary metabolism that previously were unsolv able, or that had been incompletely solved by the accumulation merely of indirect or circumstantial evidence. Urinary creatinine was shown indubitably to be derived from tissue creatine, the major mechanism for urea formation was proven to be the ornithine-arginine cycle, the conversion of phenylalanine to tyrosine was found to occur readily 257 Annu. Rev. Biochem. 1942.11:257-282. Downloaded from www.annualreviews.org Access provided by McMaster University on 02/03/15. For personal use only. Quick links to online content Further ANNUAL REVIEWS 258 MITCHELL in the body, even in the presence of an excess of dietary tyrosine, the importance of glutamic and aspartic acids in transamination was es tablished, the conversion of ornithine to proline and glutamic acid was shown to occur, r.nd the anabolic utilization of unnatural isomers by inversion to the natural form was demonstrated with d ( + ) -leucine.Furthermore, new phases in amino acid metabolism were discovered that could only have been suspected previously, such as the irreversi bility of the deamination reaction in volving lysine, as compared with the ready reversibility of the same reaction with leucine, histidine, and many other amino acids. However, the most important discovery of the researches of Schoenheimer and his colleagues with labeled amino acids was the revelation of the existence of a dynamic equilibrium be tween the tissues of the body and the surrounding nutrient media that had not previously been suspected.Schoenheimer & :Kitten berg (1) in a recent review of their work, explain the situation in the following words:The fact that the living organism, in contrast to the dead material, keeps constant the form of the cells and organs as well as the chemical structure of the large molecules has led many investigators to believe that the tissue enzymes, which show their destructive power during autolysis, lie dormant during life and are "activated" only when their functi on is required. The results obtained with isotopes make such a supposition unne...