2 FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 



Judging on the basis of such information as was available at the 

 time when the preceding was written, Voit further stated in regard 

 to the proteins : 



Die verschiedene Modificationen des Eiweisses haben hochst wahr- 

 seheinlieh sammtlich ganz die gleiehe Wirkung auf den Stoffumsatz, jedoch 

 liegen hieriiber noeh keine genauen Untersuehungen vor.* 



Certain exceptions to the assumed physiological equivalence 

 of the proteins of various origins have long been known, conspicu- 

 ously in the case of gelatin. This nitrogenous compound, possessing 

 many of the most characteristic chemical features of a typical pro- 

 tein, was found to be inadequate as the sole source of nitrogen to the 

 higher animals. Its lack of a constituent tyrosine complex and of 

 sulphur was appreciated; and to this was often charged the in- 

 sufficiency of its nutritive functions. It has remained for the newer 

 researches on the proteins those interesting studies of the past ten 

 years which have unraveled so much of the structural complexity 

 of the albuminous molecule to raise new questions and place the 

 problems of nutrition in a new light. It has been the structural dis- 

 similarity rather than the likeness of the proteins which has aroused 

 physiological comment. The rich fund which these investigations of 

 the constituent amino-acid derivatives of the various proteins of 

 both animal and plant origin have disclosed need not be presented 

 here.f The more striking findings are rapidly becoming a matter 

 of common knowledge in physiological circles. Thus it is appreciated 

 that the zein of maize and the gliadin of wheat show distinctive 

 features in respect to the yield of the different amino-acids obtain- 

 able from them. Zein is distinctly deficient in comparison with most 

 other proteins: it yields no tryptophane, glycocoll, or lysine. Glia- 

 din is characteristically rich in the glutaminic acid complex. Other 

 equally interesting illustrations might be cited. 



Are these different proteins, specific in respect to origin, biolog- 

 ically differentiated and individually unique in chemical make-up, 

 of equal value for the purposes of nutrition? If they are not, how 

 are we to explain the uniformity, apparent or real, which is exhibited 

 in the composition of the individuals of any species living under 

 widely divergent dietetic conditions ? The flesh-eater and his herbiv- 

 orous neighbor accumulate specific muscle-proteins, blood-proteins, 

 and brain-proteins despite recognized differences in the nitrogenous 

 intake. 



The problem here presented has been both simplified and com- 

 plicated by the trend of recent studies in the chemistry of digestion. 



* C. Voit: Hermann's Handbuch der Physiologie, 1881, vi (i), p. 104. 



t Elaborate reviews of this subject will be found in the monographs by Plimmer, 

 Sehryver, and T. B. Osborne, in the series of Monographs on Biochemistry, Longmans, 

 Green & Co.; also Die Pflanzenproteine, Ergebnisse der Physiologie, 1910, x, pp. 47-215. 



