642 D. B. COWIE 
members gain in radioactivity at the expense of the family “head”. These trans- 
formations are not affected by the presence of external amino acids. It can be con- 
cluded therefore that the amino acid—macromolecule complex once formed is not 
broken during the series of reactions required for the conversion of one pool amino 
acid to another. 
The schematic diagram shown in Fig. 6 is useful as an aid in presenting one inter- 
pretation of the results cited above. 
An exponentially growing cell rapidily forms from fructose, family head amino 
acids which are adsorbed on specific sites on protein surfaces. Aspartic acid, for 
example will be adsorbed on several types of sites indicated in the figure as ASP, 

Exogenous ti fe 
Methionine METH 4 
Isoleucine / ee 
Threonine : oS hiss ; 
z Ue YO 
wy Vie 
oY 4 Ya 
¥ 
[“c]Fructose Rey Y 
Y / 
Exogenous 
Aspartic acid 3 
To all empty Empty site—=— 
aspartic “family” 
it 
irate Occupied site= 
Cell wall 
Protein 
Fig. 6. Site model to explain behavior of aspartic family of amino acids. 
THRE, METH and 1so_. When adsorbed on Asp it ultimately is incorporated into 
protein as aspartic acid. Adsorbed on a METH site, however, it must be transformed 
to methionine before incorporation into protein occurs. Conversion of other aspartic 
acid molecules on sites THRE and IsoL to the amino acids threonine and isoleucine 
respectively, are also necessary before final protein utilization. 
If exogenous amino acids are also present in the medium they will compete with 
the endogenously formed amino acids (from fructose) for empty amino acid sites 
but will not displace amino acids already adsorbed. For example exogenous [!#C]- 
methionine could fill any unoccupied METH site and thus exclude @C derived from 
‘“C\fructose from reaching protein methionine. However exogenous |!*C|methionine 
would not displace either aspartic acid or methionine already adsorbed on the METH 
sites and hence could not compete with radioactive pool materials. Only when the 
pool methionine is withdrawn for protein incorporation (and a site left unoccupied) 
may another aspartic acid (or methionine) be adsorbed. 
The “site” described in the above model obviously represents a very complex 
system. For example the aspartic-family pathway shown briefly below involves a 
number of enzymatic reactions: 
References p. 645 
