604 METABOLISM, NUTRITION AND DIETETICS 



gelatin molecule. Zein, an alcohol-soluble protein or prolamin* 

 derived from maize, yields no tryptophane, glycin, or lysin. Now, 

 it is found that neither gelatin nor zein can replace the whole of 

 the ordinary proteins in the food. When only enough protein is 

 taken to prevent loss of nitrogen from the body, one-fifth of the 

 necessary nitrogen can be supplied in the form of gelatin. When 

 the food is much richer than this in ordinary protein, a correspond- 

 ingly greater proportion of the protein can be replaced by gelatin. 

 The surplus is not used in the endogenous metabolism of the cells 

 (p. 564), but supplies energy to the body after the elimination of 

 its nitrogen as urea, just as the surplus protein would do. Thus 

 gelatin economizes protein in the same way that fat and carbo- 

 hydrates do, but also to some extent in a different way by supplying 

 ' building stones ' for the protoplasm. It is therefore an interesting 

 question whether gelatin can fully replace protein when the missing 

 substances are given in addition. Kauffmann has stated that his 

 own nitrogen requirement (15-2 grammes) was almost completely 

 covered by a mixture containing 93 per cent, of the nitrogen in the 

 form of gelatin, 4 per cent, as tyrosin, 2 per cent, as cystin, and 

 i per cent, as tryptophane, in addition to the same amounts of 

 carbo-hydrate and fatty food as in the comparison diet, in which 

 the nitrogen was supplied in the form of plasmon, a commercial 

 preparation of casein. 



Similar results have been reported in experiments on animals in 

 which attempts have been made to ' complete ' such inadequate 

 proteins by addition of the missing amino-bodies, with fair but, 

 according to Osborne and Mendel, not entirely satisfactory results. 

 The converse experiment, in which an amino-acid such as trypto- 

 phane has been purposely eliminated from the food mixture, has 

 also been tried, with the result that rapid deterioration in the 

 condition of the animal ensued. It would seem, indeed, that 

 whatever capacity the animal body may have for synthesizing 

 certain of the amino-acids, this power does not extend to the cyclic 

 compounds tryptophane, tyrosin, phenylalanin, and histidin, which 

 must be supplied in the food. It has been suggested by Osborne 

 that in this regard an essential difference exists between the animal 

 and the plant, the latter alone being endowed with the function of 

 ' cyclopoiesis,' or formation of substances of the cyclic type. It 

 is not clearly understood as yet on what this difference really hinges, 

 whether, as some have supposed, on the inability of the animal 

 organism to form the appropriate fatty acid radicals, or on some 



* The prolamins are so called because on hydrolysis they yield exceptionally 

 large amounts of prolin (p. 354) and ammonia. They are insoluble in water 

 and absolute alcohol, but soluble in 70 to 80 per cent, alcohol and in dilute 

 acids and alkalies. Besides zein they include gliadin (from wheat and rye), 

 hordein (from barley), and bynin (from malt). They are extraordinarily rich 

 in glutaminic acid, hordein yielding more than any protein hitherto investi- 

 gated (over 4 1 per cent) . 



