GROWTH AND NUTRITION 



7i 



superior to either alone. We explain that fact by saying each 

 protein supplements the other. 



Since proteins differ so enormously in their nutritional or 

 biological value, efforts were initiated years ago to discover the 

 reason ; and since these substances differ, not only quantitatively 

 in the amino acids they yield, but also qualitatively in those that 

 are present, interest was directed at once to the possibility that 

 some of these amino acids may be indispensable, while others 

 are not required at all by the animal body. 



Figure 4$. Histidine is an essential amino acid. After Rose and 

 Cox. By permission of the Journal of Biological Chemistry. 



One of the earliest methods of attacking this problem was 

 that of hydrolyzing the proteins, either by enzymes or by acids, 

 and then feeding the product. Variations of this method were 

 to separate certain amino acids from the mixture and then feed 

 the remainder. Space will not permit more than mere mention 

 of workers in this field, but apparently Henriques and Han- 

 sen, over forty years ago, were pioneers. These earlier methods 

 were extended somewhat by other workers, and the net result 

 of their work would seem to show that tryptophane is indispen- 

 sable, but tyrosine is still questionable in that respect. 



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