RECENT ADVANCES IN SCIENCE 279 



amino acids it is essential to include in the mixture a minute 

 quantity of vitamine in the form of a nitrogen-free alcoholic 

 extract of fresh milk. Casein hydrolysed by boiling for forty 

 hours with 20 per cent, sulphuric acid forms a suitable amino 

 acid mixture, but is deficient in cystine and tryptophan, the 

 latter substance being destroyed by heating with acid. Accord- 

 ingly, the hydrolysis mixture has to be treated with small 

 quantities of these two acids in order to make it suitable for 

 supporting growth. It has been found that if tryptophan is 

 purposely withheld loss of body weight results almost immedi- 

 ately, and the animal dies more or less rapidly. The explana- 

 tion offered for this phenomenon is that the body is unable to 

 synthesise the indole ring requisite for the protein molecule 

 from other substances than tryptophan. Similar effects are 

 produced by the removal of the two amino acids histidine and 

 arginine, and it is argued that this means that the body is 

 unable to synthesise the guanidine and iminazole complexes 

 from other sources than these two acids. In striking contrast 

 to these observations it is found that the removal of two other 

 acids, glutamic and aspartic, has no harmful effect, and this in 

 spite of the fact that they are important constituents of casein, 

 seeing that they make up no less than 28 per cent, of the weight 

 of the molecule. It may however be assumed that the body is 

 in this case capable of making up the deficiency in such com- 

 paratively simple molecular structures by synthesis from fats 

 or carbohydrates in the presence of ammonium salts, and that 

 the synthesis can be maintained at a sufficient rate to keep 

 pace with the requirements of the body. As illustrating the 

 fact that one amino acid may to a certain extent replace another 

 closely allied acid it has been found that growth could be 

 maintained on an amino acid mixture containing no tyrosine 

 but a small quantity of phenylalanine, which differs from the 

 former acid only by a single atom of oxygen, as may be seen 

 from the formulae 



C 6 H 4 OH.CH 2 CHNH 2 COOH and C 6 H 5 CH 2 .CHNH 2 COOH 



Tyrosine. Phenylalanine. 



On the other hand gelatine, which contains neither tryptophan 

 nor tyrosine, fails entirely to support life, but the fact that 

 on addition of tryptophan life could be maintained points to 

 the fact that the body contains some mechanism for synthesising 



