Geiling (16) digested casein of milk to the amino-acid stage and then precipitated the arginin, histidin and lysin with phosphotungstic acid as Henrique and Hansen (10) had done, and conducted experiments in feeding the portion of the protein without these diamino-acids. Mice were used as experimental animals. His diets also contained "protein-free milk," which, as will be shown later, contained protein nitrogen (amino-acids) sufficient to vitiate in great measure the results of his experiments. Geiling interpreted his findings as indicating that arginin and histidin are interconvertible in the body. He also concluded that lysin is not necessary for maintenance without growth. These observations were confirmatory of studies made by Ackroyd and Hopkins (17) and by Osborne and Mendel (18).

71. The Transmutability Of Certain Amino-Acids Into Each Other

Ackroyd and Hopkins (17) had carried out feeding experiments with mice fed the essential non-nitrogenous nutrients supplemented with simple mixtures of purified amino-acids. Their conclusion was that if arginin is present in the diet histidin may be omitted, and vice versa. A similar relationship they found between tyrosin and phenylalanin, the structural formulas of which are closely similar. This was the first instance where it was believed that it had been demonstrated that one amino-acid may be converted into another.