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The  Relative  Value  of  Proteins.       \ Am^  J<Jur-15Sarm- 
'  (      Sept.,  1921. 
amino-acids  may  be  indispensable,  others  not  so  essential.  There  are 
two  main  problems  to  study  in  nutrition :  the  formation  of  new  tis- 
sue, as  in  the  growth  of  young  animals,  and  the  maintenance  of 
tissue,  which  undergoes  so-called  wear  and  tear,  in  adult  animals. 
The  problem  is  ultimately  to  find  out  the  function  of  each  amino- 
acid  in  growth  and  maintenance. 
THE  EFFECT  OF  THE  ABSENCE  OF  CERTAIN  AMINO-ACIDS. 
The  most  certain  way  of  studying  the  problem  is  to  feed  ani- 
mals upon  known  mixtures  of  amino-acids,  but  the  practical  diffi- 
culties are  far  too  great.  The  amino-acids  are  not  easily  prepared, 
and  it  is  almost  impossible  to  obtain  sufficient  of  each  of  them  to 
feed  an  animal,  even  a  mouse,  for  any  length  of  time.  Two  other 
ways  are  possible: — (a)  To  feed  incomplete  proteins  and  add  the 
missing  unit  or  units ;  (b)  to  feed  completely  hydrolysed  proteins, 
i.  e.j  a  mixture  of  amino-acids  from  which  one  or  more  units  have 
been  removed  by  chemical  means. 
The  first  experiment  of  this  kind  was  made  by  Willcock  and 
Hopkins  in  1906.  Zein  was  chosen  as  incomplete  protein  and  fed  to 
mice:  in  one  set  alone,  in  another  set  with  the  addition  of  2  per 
cent,  of  its  amount  of  tryptophan.  Failure  occurred  in  both  sets  of 
animals,  but  not  so  rapidly  in  those  on  zein  and  tryptophan.  Young 
mice  with  zein  alone  died  generally  in  16  days ;  with  zein  and  trypto- 
phan in  30  days.  Adult  mice  without  tryptophan  lived  27  days,  with 
tryptophan  49  days.  The  survival  period  was  thus  appreciably 
lengthened  by  the  presence  of  tryptophan.  The  failure  to  live  was 
most  probably  due  to  the  absence  of  other  units  from  the  zein. 
Ackroyd  and  Hopkins  repeated  the  experiment  in  1916  by  the  second 
method  of  experimenting  which  offers  better  conditions.  The  ani- 
mals were  fed  upon  a  mixture  of  the  amino-acids  from  casein.  This 
mixture  does  not  contain  tryptophan,  since  it  is  destroyed  by  acid 
hydrolysis.  In  the  first  period  tryptophan  was  added ;  on  the  twelfth 
day  it  was  omitted  and  introduced  once  more  on  the  thirty-fifth 
day.  The  animals  continued  their  growth  during  the  first  period, 
declined  in  weight  during  the  second  period,  and  grew  again  in  the 
third  period  when  the  tryptophan  was  present. 
Osborne  and  Mendel  in  America  have  made  numerous  experi- 
ments with  various  pure  isolated  proteins  of  known  amino-acid 
composition.    Their  most  important  results  in  this  connection  were 
