8. P R O T E I N M E T A B O L I S M IN C E L L L I N E S
283
less effectively by other carbohydrates, reflects the fact that the cell's
energy requirements are drawn principally from this source. Certain
cell lines require pyruvate, a requirement which is paradoxical in the
same sense as the cystine requirement, in that the cell produces large
amounts of lactate, presumably by way of pyruvate. Like the cystine
requirement, the need for exogenous pyruvate disappears at sufficiently
high population densities.
The apparent requirement by most cells for serum protein, either as a
macromolecule, per se, or as a carrier of smaller molecules, is discussed
in a following section.
D . C O M P O U N D S N O T R E Q U I R E D B Y C U L T U R E D C E L L S
It is evident that there are a number of compounds, the absence of
which leads to disease in the whole animal, but which are apparently
not required by serially cultured cells for survival and growth. T w o
general explanations may be offered. Such compounds may be present
as trace contaminants in other constituents of the medium (particularly
the serum protein),in amounts sufficient for growth, therefore preventing
the demonstration of a nutritional requirement. Alternatively, a particular factor may be necessary for the specialized function of an organ
or cell system in vivo, but not for the survival and growth of "dedifferentiated" cells in vitro. Biotin may be an example of a nutritionally
essential compond whose role cannot be demonstrated because of contamination; while ascorbic acid, the fat-soluble vitamins, and possibly
vitamin B 12 may be examples of vitamins necessary for specialized
organs, but which do not play a necessary role in the metabolism of
cultured cells.
The relative simplicity of the minimal medium emphasizes the fact
that there are additionally a large number of compounds which are
metabolically essential, in the sense that they are indispensable building
blocks, intermediates, or catalysts, but which need not be exogenously
supplied to permit cellular propagation. Some of these substances may
be provided in minute but sufficient amounts as contaminants of other
constituents of the medium, but most are produced by the cell from
the minimum essential compounds of Table I.
Typical cultured cells can produce all their necessary alanine, serine
and glycine (largely from glucose) and asparagine, aspartate, and proline (from glutamine via glutamate (cf. p. 289)). Adequate synthesis
of the coenzymes, such as DPN, TPN, CoA, and pyridoxal phosphate
from their vitamin moieties apparently is a general property of cultured cells, since the coenzymes are usually no more effective as growth
factors than their constituent vitamins.
283
less effectively by other carbohydrates, reflects the fact that the cell's
energy requirements are drawn principally from this source. Certain
cell lines require pyruvate, a requirement which is paradoxical in the
same sense as the cystine requirement, in that the cell produces large
amounts of lactate, presumably by way of pyruvate. Like the cystine
requirement, the need for exogenous pyruvate disappears at sufficiently
high population densities.
The apparent requirement by most cells for serum protein, either as a
macromolecule, per se, or as a carrier of smaller molecules, is discussed
in a following section.
D . C O M P O U N D S N O T R E Q U I R E D B Y C U L T U R E D C E L L S
It is evident that there are a number of compounds, the absence of
which leads to disease in the whole animal, but which are apparently
not required by serially cultured cells for survival and growth. T w o
general explanations may be offered. Such compounds may be present
as trace contaminants in other constituents of the medium (particularly
the serum protein),in amounts sufficient for growth, therefore preventing
the demonstration of a nutritional requirement. Alternatively, a particular factor may be necessary for the specialized function of an organ
or cell system in vivo, but not for the survival and growth of "dedifferentiated" cells in vitro. Biotin may be an example of a nutritionally
essential compond whose role cannot be demonstrated because of contamination; while ascorbic acid, the fat-soluble vitamins, and possibly
vitamin B 12 may be examples of vitamins necessary for specialized
organs, but which do not play a necessary role in the metabolism of
cultured cells.
The relative simplicity of the minimal medium emphasizes the fact
that there are additionally a large number of compounds which are
metabolically essential, in the sense that they are indispensable building
blocks, intermediates, or catalysts, but which need not be exogenously
supplied to permit cellular propagation. Some of these substances may
be provided in minute but sufficient amounts as contaminants of other
constituents of the medium, but most are produced by the cell from
the minimum essential compounds of Table I.
Typical cultured cells can produce all their necessary alanine, serine
and glycine (largely from glucose) and asparagine, aspartate, and proline (from glutamine via glutamate (cf. p. 289)). Adequate synthesis
of the coenzymes, such as DPN, TPN, CoA, and pyridoxal phosphate
from their vitamin moieties apparently is a general property of cultured cells, since the coenzymes are usually no more effective as growth
factors than their constituent vitamins.