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The  Relative  Value  of  Proteins. 
5  Am.  Jour.  Pharm. 
I      Sept.,  1921. 
Cystine  is  the  only  unit  which,  contains  the  element  sulphur, 
though  another  sulphur-containing  compound  may  be  present  in 
proteins.  The  amount  of  cystine  is  not  known  except  in  a  few 
cases,  but  it  is  estimated  from  the  sulphur  content.  The  need  of 
cystine  in  the  food  has  been  shown  most  conclusively  by  experiments 
with  phaseolin,  the  protein  of  the  navy  bean.  Slow  growth  resulted 
on  a  diet  with  the  protein  alone,  but  normal  growth  followed  the 
addition  of  2  per  cent,  of  its  amount  of  cystine.  Very  little  sulphur 
is  present  in  casein.  Osborne  and  Mendel  found  that  less  casein 
was  required  in  a  diet  if  it  were  augmented  with  cystine:  15  per 
cent,  casein  alone  gave  normal  growth,  but  9  per  cent,  if  cystine 
were  added. 
It  is  not  possible  to  test  a  protein  without  proline,  but  arachin 
with  only  1.4  per  cent,  was  tried  by  Sure.  This  protein  failed  tc 
give  normal  growth  even  after  the  addition  of  extra  proline,  so  that 
its  deficiency  must  be  caused  by  another  missing  substance.  It  is 
possible  that  proline  and  glutamic  acid  are  related  units  in  the 
molecules  of  the  proteins. 
An  experiment  has  also  been  made  to  see  if  the  animal  organ- 
ism can  introduce  an  amino  group  into  the  molecule  of  nor-leucine 
and  convert  it  into  lysine.  Animals  fed  upon  gliadin  and  nor- 
leucine  did  not  grow  just  as  on  gliadin  alone.  The  synthesis  is  thus 
not  possible. 
The  whole  group  of  simple  mono-amino-acids  has  yet  to  be 
tested.  They  may  not  all  be  necessary.  In  cases  of  diabetes  several 
can  give  rise  to  glucose;  their  function  may  be  to  supply  energy 
through  their  conversion  into  sugar.  Isoleucine  is  absent  from 
gelatin,  as  recently  shown  by  Dakin ;  this  may  be  another  reason  for 
the  failure  of  gelatin  in  nutrition,  which  is  usually  attributed  to  the 
absence  of  tyrosine,  cystine  and  tryptophan. 
*  COMPARATIVE    NUTRITIVE   VALUE   OF  PROTEINS. 
Few  proteins  show  such  marked  deficiencies  as  gelatin,  gliadin 
and  zein.  The  different  amounts  of  the  units  in  complete  proteins 
make  little  difference  to  growth  if  the  diet  contain  abundance  of  pro- 
tein. Growth  is  observed  on  the  most  varied  proteins  of  animal 
and  plant  origin;  but  if  any  restriction  in  the  amount  of  protein  in 
the  food  be  made,  then  the  growth  is  lessened  or  inhibited.  Each 
complete  protein  will  thus  have  a  definite  minimum  for  growth. 
