8. P R O T E I N M E T A B O L I S M IN C E L L LINES
287
I I I .
A M I N O A C I D A N D P R O T E I N M E T A B O L I S M
IN C E L L C U L T U R E S
A. A M I N O A C I D SYNTHESIS
O f the thirteen amino acids required by most cultured cells, cystine
is in fact synthesized from methionine and glucose, apparently by the
classic pathway involving the biosynthesis of homocysteine and serine
from methionine and glucose respectively, their condensation to
cystathionine, and the cleavage of the latter to cysteine and homoserine.
As has been discussed, the requirement for cyst(e)ine is populationdependent, and disappears at sufficiently high population densities
(Eagle, Piez and Oyama, 1961). The critical population density, at
which the synthesized cystine suffices for cellular growth, is a function
of the specific precursors provided (Table I I ) . Given only methionine
and glucose, the minimum population consistent with cellular survival
and growth is 200,000-500,000 cells/ml; at the other extreme, given
both homocystine and serine as precursors, the critical population
density is 50-500 cells/ml.
Although arginine can be synthesized from citrulline, which will
substitute for arginine in the minimal medium of Table I, the rest of the
ornithine cycle is not operative in cell culture and the cells cannot
utilize ornithine for arginine synthesis. The source of the urea formed
by the cell has not been investigated specifically in cell cultures.
Glutamine serves in cultured cells as a specific precursor both of
protein and nucleic acid, and it thus plays a central role in the metabolic scheme. The fact that the cells can be adapted to growth in a
medium containing glutamic acid instead of glutamine does not affect
the total number of amino acids required. Cells which ordinarily
derive their glutamic acid from preformed glutamine are able after
such adaptation to derive glutamine from glutamic acid, presumably
utilizing the a-amino group of glutamic acid as the source of the glutamine amide group.
When cells are grown in the minimal medium of Table I and are
then under the necessity of synthesizing the remaining six amino acids
(alanine, serine, glycine, aspartic acid, asparagine and proline) the
first three derive their carbon skeletons primarily from glucose, and the
last three primarily from glutamine (or glutamic acid) (cf. Table I I I ) .
In the glucose family of amino acids, all the glycine is apparently
synthesized by way of serine, with relatively minor conversion of
glycine to serine. Further, there is negligible metabolic interconversion
287
I I I .
A M I N O A C I D A N D P R O T E I N M E T A B O L I S M
IN C E L L C U L T U R E S
A. A M I N O A C I D SYNTHESIS
O f the thirteen amino acids required by most cultured cells, cystine
is in fact synthesized from methionine and glucose, apparently by the
classic pathway involving the biosynthesis of homocysteine and serine
from methionine and glucose respectively, their condensation to
cystathionine, and the cleavage of the latter to cysteine and homoserine.
As has been discussed, the requirement for cyst(e)ine is populationdependent, and disappears at sufficiently high population densities
(Eagle, Piez and Oyama, 1961). The critical population density, at
which the synthesized cystine suffices for cellular growth, is a function
of the specific precursors provided (Table I I ) . Given only methionine
and glucose, the minimum population consistent with cellular survival
and growth is 200,000-500,000 cells/ml; at the other extreme, given
both homocystine and serine as precursors, the critical population
density is 50-500 cells/ml.
Although arginine can be synthesized from citrulline, which will
substitute for arginine in the minimal medium of Table I, the rest of the
ornithine cycle is not operative in cell culture and the cells cannot
utilize ornithine for arginine synthesis. The source of the urea formed
by the cell has not been investigated specifically in cell cultures.
Glutamine serves in cultured cells as a specific precursor both of
protein and nucleic acid, and it thus plays a central role in the metabolic scheme. The fact that the cells can be adapted to growth in a
medium containing glutamic acid instead of glutamine does not affect
the total number of amino acids required. Cells which ordinarily
derive their glutamic acid from preformed glutamine are able after
such adaptation to derive glutamine from glutamic acid, presumably
utilizing the a-amino group of glutamic acid as the source of the glutamine amide group.
When cells are grown in the minimal medium of Table I and are
then under the necessity of synthesizing the remaining six amino acids
(alanine, serine, glycine, aspartic acid, asparagine and proline) the
first three derive their carbon skeletons primarily from glucose, and the
last three primarily from glutamine (or glutamic acid) (cf. Table I I I ) .
In the glucose family of amino acids, all the glycine is apparently
synthesized by way of serine, with relatively minor conversion of
glycine to serine. Further, there is negligible metabolic interconversion