A. G. Hogan | 13 
while according to Johns and Brewster (1916) kafirin contains 
only 0.6 per cent of that element. It is an assumption to cal- 
culate that all the sulfur of these proteins is cystine sulfur, but 
the data make it seem possible that the amount of cystine in 
kafirin is well below the optimum. 
The statement is frequently encountered in the literature 
that tyrosine is an essential amino-acid. Totani (1916) believes 
that tyrosine is not necessarily essential, and apparently the 
facts in the case have not been finally established. 
In an effort to investigate the deficiencies of kafirin more 
definitely, the amino-acids? mentioned were added directly to 
the basal rations. These additions were made singly or collec- 
tively as follows: lysine, cystine, lysine plus cystine, lysine plus 
tyrosine, lysine plus tryptophane. 
-The lot receiving the basal ration plus lysine grew slowly, in 
accordance with our expectations. Evidently lysine (Chart 2) 
is the first limiting factor in kafirm. Accordingly, in our other 
feeding trials (Chart 3), lysine was always made a part of the 
basal ration. The further addition of tryptophane to the diet 
did not increase the rate of growth, and a similar addition of tyro- 
sine (Chart 4) was equally ineffective. When cystine was super- 
imposed on this ration, however, the animals grew somewhat 
more rapidly. Small variations might seem insufficient to estab- 
lish this point, but the unanimity of the results seems conclu- 
sive. Furthermore, the marked acceleration in growth that 
follows the addition of cystine after it has been withheld for 
some time seems even more conclusive. Accordingly, our in- 
terpretation of the data is that cystine is the second limiting 
factor in kafirin. Charts 3 and 4 give the data in greater detail. 
2TIn some cases the tyrosine used was a Kahlbaum preparation, in 
others it was prepared by the author. The cystine, tryptophane, and 
lysine were prepared in this laboratory. 
The amount of lysine added varied from 3 to 6 per cent of the kafirin 
in the ration, but the larger amount seemed no more effective than the 
smaller. The cystine formed 5 per cent of the kafirin, and enough tyrosine 
was added to equal 4 per cent of the protein. 
