7. C A R B O H Y D R A T E A N D E N E R G Y M E T A B O L I S M
249
3. Utilization of other Energy-yielding Reactions
While carbohydrate and especially glucose are undoubtedly the main
source of energy in normal circumstances there is evidence that cultured
cells can also derive energy from amino acids after deamination.
Holmes and Watchorn (1927) and Warburg and Kubowitz (1927)
demonstrated that cultures of kidney would produce ammonia and that
glucose had an amino-acid sparing effect similar to that in the whole
animal. Similar observations have been made with other kinds of
cultured tissue (Eagle, 1959),, and Barban and Schulze (1959) have
demonstrated transaminating enzymes in extracts of HeLa cells and
mouse fibroblasts. Our own unpublished observation that respiration
can be maintained much longer in the absence of glucose if amino
acids are present in the medium also suggests that amino acids can
contribute to the cell's energy requirements. In some investigations
fairly extensive degradation of serum proteins in the culture medium
has been described (Jordan et al., 1959; Westfall, Peppers and Earle,
1955; Kent and Gey, 1957) and the amino acids from this source may
presumably be utilized also.
Cultured cells are fairly rich in lipids (Schaffer, Grunbaum and Kirk,
1954; Paul, 1958). Little is known about the metabolism of this material
but undoubtedly in certain conditions lipid droplets can appear in cells
and if the conditions are changed they can disappear again (Grunbaum, Schaffer and Kirk, 1955; Lewis, 1922). Without more detailed
study it is not possible to exclude the possibility that some lipid may be
actively excreted. However, since cells can certainly synthesize lipids
(Berliner, Swim and Dougherty, 1958; Halevy and Geyer, 1961) it
seems reasonably likely that they may also degrade them and utilize
them as an energy source. However, a recent paper (Rose, Maca and
Pace, 1961) indicates that at least some lipid is actively excreted and
not utilized.
4. Connexions with other Metabolic Pathways
There is ample evidence that cells in culture are capable of performing most of the conversions displayed by intact fresh tissues and tissue
extracts. The connexions with amino acid and nucleic acid synthesis
are particularly important.
The ability of cells to grow in a medium, such as Eagle's (1959),
which contains an incomplete amino acid supplement, implies that they
are able to synthesize the other amino acids by transamination to
carbon skeletons derived mainly from carbohydrate pathways. More
direct evidence for this was obtained by Fischer and his colleagues
(1952) who found that
1 4
C from glucose could be recovered from
249
3. Utilization of other Energy-yielding Reactions
While carbohydrate and especially glucose are undoubtedly the main
source of energy in normal circumstances there is evidence that cultured
cells can also derive energy from amino acids after deamination.
Holmes and Watchorn (1927) and Warburg and Kubowitz (1927)
demonstrated that cultures of kidney would produce ammonia and that
glucose had an amino-acid sparing effect similar to that in the whole
animal. Similar observations have been made with other kinds of
cultured tissue (Eagle, 1959),, and Barban and Schulze (1959) have
demonstrated transaminating enzymes in extracts of HeLa cells and
mouse fibroblasts. Our own unpublished observation that respiration
can be maintained much longer in the absence of glucose if amino
acids are present in the medium also suggests that amino acids can
contribute to the cell's energy requirements. In some investigations
fairly extensive degradation of serum proteins in the culture medium
has been described (Jordan et al., 1959; Westfall, Peppers and Earle,
1955; Kent and Gey, 1957) and the amino acids from this source may
presumably be utilized also.
Cultured cells are fairly rich in lipids (Schaffer, Grunbaum and Kirk,
1954; Paul, 1958). Little is known about the metabolism of this material
but undoubtedly in certain conditions lipid droplets can appear in cells
and if the conditions are changed they can disappear again (Grunbaum, Schaffer and Kirk, 1955; Lewis, 1922). Without more detailed
study it is not possible to exclude the possibility that some lipid may be
actively excreted. However, since cells can certainly synthesize lipids
(Berliner, Swim and Dougherty, 1958; Halevy and Geyer, 1961) it
seems reasonably likely that they may also degrade them and utilize
them as an energy source. However, a recent paper (Rose, Maca and
Pace, 1961) indicates that at least some lipid is actively excreted and
not utilized.
4. Connexions with other Metabolic Pathways
There is ample evidence that cells in culture are capable of performing most of the conversions displayed by intact fresh tissues and tissue
extracts. The connexions with amino acid and nucleic acid synthesis
are particularly important.
The ability of cells to grow in a medium, such as Eagle's (1959),
which contains an incomplete amino acid supplement, implies that they
are able to synthesize the other amino acids by transamination to
carbon skeletons derived mainly from carbohydrate pathways. More
direct evidence for this was obtained by Fischer and his colleagues
(1952) who found that
1 4
C from glucose could be recovered from