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
261
optimum for most cell-types seems to be around ordinary atmospheric
oxygen tension (0-2 atm).
4. Requirement for Carbon Dioxide
One of the least satisfactory practical aspects of tissue culture is that
the ultimate buffering system is usually carbonic acid/bicarbonate,
which is in equilibrium with carbon dioxide in the gas phase. This is
both inconvenient to use and not as effective as one might wish.
However, attempts to dispense with C0 2 -buffers have revealed that
carbon dioxide is essential for cellular growth. Studies aimed at
investigating this point (Geyer and Chang, 1958; Geyer and Neimark,
1958; M . Harris, 1954; Swim and Parker, 1958; Whitfield and Rixon,
1961) are virtually unanimous. Cellular respiration is inhibited in the
absence of carbon dioxide (Bicz, 1960; Danes et al., 1963; Danes and
Kieler, 1958; Kieler, 1960a, b) and this has led to the suggestion that
decarboxylation of oxaloacetic acid occurs readily when C O a is absent.
A requirement for purine and pyrimidine synthesis has also been
suggested (Chang, Liepins and Margolish, 1961). The former explanation seems to account for most phenomena of carbon dioxide deprivation and is supported by the observation that cells will grow in a C O 2 -
free medium if oxaloacetic acid is added (Gwatkin and Siminovitch,
1960). This effect is somewhat temporary since oxaloacetate is rapidly
degraded to pyruvate and has therefore to be renewed frequently. W e
have employed a slightly different solution to this problem by using a
tris-citrate buffering system in which several cell lines have been
maintained in continuous cultivation for some months (Paul et al.,
1965). This was developed on the hypothesis that the depletion of
oxaloacetic acid inhibits the formation of citric acid (which is usually
formed by condensation of oxaloacetic acid and acetyl coenzyme A) and
consequently leads to an inhibition of the Krebs cycle.
5. Other Environmental Factors
Certain other general environmental factors which affect cell behaviour have not been found to exert profound influence on the pattern
of carbohydrate metabolism. Thus, although variations of osmotic
pressure may influence the growth-rate considerably (Paul et al., 1965),
the metabolic pattern is little altered. This observation suggests, of
course, that environmental factors may be more important in dictating
the pattern of carbohydrate metabolism than the actual requirement
for growth or synthesis.
Growth-rate is also affected by the temperature at which cells are
cultivated (Paul et al., 1965) but there is no direct correlation with the
metabolic pattern, which tends to become progressively more glycolytic
261
optimum for most cell-types seems to be around ordinary atmospheric
oxygen tension (0-2 atm).
4. Requirement for Carbon Dioxide
One of the least satisfactory practical aspects of tissue culture is that
the ultimate buffering system is usually carbonic acid/bicarbonate,
which is in equilibrium with carbon dioxide in the gas phase. This is
both inconvenient to use and not as effective as one might wish.
However, attempts to dispense with C0 2 -buffers have revealed that
carbon dioxide is essential for cellular growth. Studies aimed at
investigating this point (Geyer and Chang, 1958; Geyer and Neimark,
1958; M . Harris, 1954; Swim and Parker, 1958; Whitfield and Rixon,
1961) are virtually unanimous. Cellular respiration is inhibited in the
absence of carbon dioxide (Bicz, 1960; Danes et al., 1963; Danes and
Kieler, 1958; Kieler, 1960a, b) and this has led to the suggestion that
decarboxylation of oxaloacetic acid occurs readily when C O a is absent.
A requirement for purine and pyrimidine synthesis has also been
suggested (Chang, Liepins and Margolish, 1961). The former explanation seems to account for most phenomena of carbon dioxide deprivation and is supported by the observation that cells will grow in a C O 2 -
free medium if oxaloacetic acid is added (Gwatkin and Siminovitch,
1960). This effect is somewhat temporary since oxaloacetate is rapidly
degraded to pyruvate and has therefore to be renewed frequently. W e
have employed a slightly different solution to this problem by using a
tris-citrate buffering system in which several cell lines have been
maintained in continuous cultivation for some months (Paul et al.,
1965). This was developed on the hypothesis that the depletion of
oxaloacetic acid inhibits the formation of citric acid (which is usually
formed by condensation of oxaloacetic acid and acetyl coenzyme A) and
consequently leads to an inhibition of the Krebs cycle.
5. Other Environmental Factors
Certain other general environmental factors which affect cell behaviour have not been found to exert profound influence on the pattern
of carbohydrate metabolism. Thus, although variations of osmotic
pressure may influence the growth-rate considerably (Paul et al., 1965),
the metabolic pattern is little altered. This observation suggests, of
course, that environmental factors may be more important in dictating
the pattern of carbohydrate metabolism than the actual requirement
for growth or synthesis.
Growth-rate is also affected by the temperature at which cells are
cultivated (Paul et al., 1965) but there is no direct correlation with the
metabolic pattern, which tends to become progressively more glycolytic