8. P R O T E I N M E T A B O L I S M IN CELL LINES
281
Cyst(e)ine is an example of a paradoxical nutritional requirement
for an amino acid which the cell has the capacity to synthesize. As
discussed in a following section, this cystine requirement disappears
above a critical population density, and is referable quite simply to
the loss of the newly synthesized amino acid to the surrounding medium.
The foregoing considerations make it apparent that the requirement
of cells in vitro for arginine, tyrosine and cyst(e)ine does not necessarily
imply a difference in metabolic capacities between such cultured cells
and cells in vivo. The explanation for the requirement for glutamine
(or glutamic acid) and for histidine is not presently clear. A number
of experimental animals, as well as human infants, do in fact require
histidine for nitrogen balance (Meister, 1957; Snyderman, Holt,
Norton, Smellie and Boyer, 1957), and a possible biosynthetic role of the
intestinal flora in the human adult has not been excluded. In the light
of the foregoing, it appears likely that the inability to synthesize
histidine may not prove to be limited to cultured cells.
Individual cell lines, or typical cells under special conditions, exhibit
exceptional requirements for additional amino acids. In most of these
instances the requirement is caused, not by a lack of the particular
biosynthetic pathway, but by conditions that prevent accumulation
of adequate intracellular levels of the specific amino acid (cf. p. 285).
B. V I T A M I N R E Q U I R E M E N T S
Only eight vitamins have been demonstrated to be required by
cultured cells, and as in the case of the amino acid requirements, there
are no striking differences between different cell strains. There appears
to be a general requirement for folic acid, nicotinamide, pantothenate,
pyridoxal, riboflavin, thiamine and choline, and most cell lines require
inositol as well. The minimum concentration of these vitamins which
permits the cells to grow at a maximal rate is of the order of 1 0 ~
7 - 1 0
-8
g/ml.
The determination of the vitamin requirements of cultured cells
is complicated by the extremely low concentrations at which they are
effective, the large intracellular reserves, and the significant contamination introduced with the serum protein. It is therefore possible that
additional vitamins will eventually be shown to be essential, despite
the fact that no such requirement has emerged from studies of the past
8 years.
The first six vitamins listed above are constituents of coenzymes
which play an essential role in cellular metabolism, and it is not
surprising that the indispensability of these factors is unequivocal in
both intact animals and cultured cells. The role of choline in the
281
Cyst(e)ine is an example of a paradoxical nutritional requirement
for an amino acid which the cell has the capacity to synthesize. As
discussed in a following section, this cystine requirement disappears
above a critical population density, and is referable quite simply to
the loss of the newly synthesized amino acid to the surrounding medium.
The foregoing considerations make it apparent that the requirement
of cells in vitro for arginine, tyrosine and cyst(e)ine does not necessarily
imply a difference in metabolic capacities between such cultured cells
and cells in vivo. The explanation for the requirement for glutamine
(or glutamic acid) and for histidine is not presently clear. A number
of experimental animals, as well as human infants, do in fact require
histidine for nitrogen balance (Meister, 1957; Snyderman, Holt,
Norton, Smellie and Boyer, 1957), and a possible biosynthetic role of the
intestinal flora in the human adult has not been excluded. In the light
of the foregoing, it appears likely that the inability to synthesize
histidine may not prove to be limited to cultured cells.
Individual cell lines, or typical cells under special conditions, exhibit
exceptional requirements for additional amino acids. In most of these
instances the requirement is caused, not by a lack of the particular
biosynthetic pathway, but by conditions that prevent accumulation
of adequate intracellular levels of the specific amino acid (cf. p. 285).
B. V I T A M I N R E Q U I R E M E N T S
Only eight vitamins have been demonstrated to be required by
cultured cells, and as in the case of the amino acid requirements, there
are no striking differences between different cell strains. There appears
to be a general requirement for folic acid, nicotinamide, pantothenate,
pyridoxal, riboflavin, thiamine and choline, and most cell lines require
inositol as well. The minimum concentration of these vitamins which
permits the cells to grow at a maximal rate is of the order of 1 0 ~
7 - 1 0
-8
g/ml.
The determination of the vitamin requirements of cultured cells
is complicated by the extremely low concentrations at which they are
effective, the large intracellular reserves, and the significant contamination introduced with the serum protein. It is therefore possible that
additional vitamins will eventually be shown to be essential, despite
the fact that no such requirement has emerged from studies of the past
8 years.
The first six vitamins listed above are constituents of coenzymes
which play an essential role in cellular metabolism, and it is not
surprising that the indispensability of these factors is unequivocal in
both intact animals and cultured cells. The role of choline in the