Diet and plasma protein composition.

From time to time it has been claimed that the composition of tissue proteins can be altered by changing the proportions of the amino acids made available to the organism. Most of these results, however, are capable of an alternative explanation. For example, Csonka, Denton & Ringel (1947) and Csonka (1950) supplemented the diet of hens with casein and obtained heavier eggs with a higher than average content of methionine and cystine; supplements of methionine and cystine did not have this effect. The authors pointed out that this result could be due either to an alteration in the composition of ‘egg protein’ or to an alteration in the relative proportions of the different proteins of the egg. They subsequently demonstrated electrophoretically (Csonka & Jones, 1952) that the higher-protein diet of the hens caused a rise in the content of egg-white globulins and a fall in that of ovomucoid. Similar findings have been reported and similar possible explanations put forward in the microbiological field. Cohen (1958) compared the performance of a valinesensitive strain of Escherichia coli with that of a valine-resistant strain and showed that valine prevented exponential growth in the former, and that its proteins contained more valine and less methionine than did those of the latter strain. The author suggested that valine had replaced other amino acids in the protein, but an alteration in the proportions of the proteins could equally well explain the results. Similarly, the difference in amino acid composition of total cellular proteins of Mycobacterium tuberculosis observed by Rao & Wadhwani (1956) on different media may have been due to a change in the proportions of the different proteins. Freeland & Gale (1947), Camien, Salle & Dunn (1945), Stokes & Gunness (1946) and Work (1949) were unable to alter the amino acid composition of bacterial protein by changing the media on which the organisms were grown. Against the background of these results the suggestion of Albanese (1953) that there was a change in the blood protein of babies when they were fed on a low-lysine diet, the lysine being replaced by arginine, stands out as unique. There were further reasons for re-examining this finding. Albanese did not analyse the blood proteins for lysine but determined only arginine and total nitrogen. Moreover, there were reasons to doubt the accuracy of the arginine determinations. Therefore rats were given diets low in lysine (diets similar to those used by Albanese) and diets free from lysine, and the arginine and lysine contents of their blood proteins were determined. * Present address : Research Department, Farley’s Infant Food Ltd, Colnbrook, Bucks. t Present address : Human Nutrition Research Unit, Nutrition Building, National Institute for Medical Research, Mill Hill, London, N.W. 7.