V. DETERMINING FACTORS IN CELL GROWTH
217
the growth induction occurs. If, however, the same inhibitors are applied
much later, after the growth induction has taken effect and the cells are
already in their rapid phase of growth, these two inhibitors are much
less effective. Work in this laboratory has shown that concentrations of
cyanide which would suppress the respiration of quiescent carrot tissue
will fail to stop the growth of cells already growing. Furthermore, the
degree to which the suppression of respiration and growth by carbon
monoxide is light reversible is also different for the pre- and postinduction systems. This merely indicates that rapidly dividing cells
manipulate their respiratory processes and the utilization of the energy
so released, in a different manner from the cells which have ceased to
grow. (This is also shown by unpublished work of one of us (F.C.S.)
together with Dr. H. Schneiderman).
Cells which have temporarily ceased to grow may become rich in
storage products, both nitrogenous and carbohydrate. Both carrot and
potato cells are rich in alcohol-soluble nitrogen compounds, and these
contain much free amino acid and also amide nitrogen in the form of
asparagine and glutamine. A speedy consequence of the growth stimulus
is the rapid conversion of these soluble nitrogen compounds into protein;
this is also true in cells of tumours (Neish and Hibbert, 1943). As this
recrudescence of protein synthesis occurs in dividing cells, the relative
composition of the residual soluble nitrogen in the tissue, which falls
to a very low level, changes quite markedly. Whereas asparagine is
conspicuous in the resting cells, it disappears almost entirely from the
growing cells, while glutamine tends to persist in the growing system.
Thus the balance shifts from a high soluble nitrogen reserve in which
asparagine may be prominent to a low soluble nitrogen reserve in which
alanine and glutamine tend to be prominent and, as these soluble
reserves are continually depleted by growth, they are replenished either
from the casein hydrolysate supplied with the external medium, or by
direct synthesis from nitrate nitrogen (Steward and Pollard, 1957;
Steward, Thompson and Pollard, 1958).
Meanwhile, however, the total protein has markedly increased in the
cells stimulated into growth; but, of greater significance than the mere
increment of total protein, is a change in its over-all composition. This
was first recognized by the presence of hydroxyproline, in amounts far
higher than expected, in the hydrolysate of the alcohol insoluble protein
of tissues (carrot root and potato tuber) grown after treatment with
coconut milk. Moreover, other rapidly proliferating cell systems such
as the spontaneous tumours on certain hybrids of tobacco, or the
cells stimulated into rapid growth by the effect of the crown gall organism on Kalanchoe or Bryophyllum (Steward et al., 1955), and even
callus from leaf tissues of Agave tourney ana (Weinstein et al., 1959) were
217
the growth induction occurs. If, however, the same inhibitors are applied
much later, after the growth induction has taken effect and the cells are
already in their rapid phase of growth, these two inhibitors are much
less effective. Work in this laboratory has shown that concentrations of
cyanide which would suppress the respiration of quiescent carrot tissue
will fail to stop the growth of cells already growing. Furthermore, the
degree to which the suppression of respiration and growth by carbon
monoxide is light reversible is also different for the pre- and postinduction systems. This merely indicates that rapidly dividing cells
manipulate their respiratory processes and the utilization of the energy
so released, in a different manner from the cells which have ceased to
grow. (This is also shown by unpublished work of one of us (F.C.S.)
together with Dr. H. Schneiderman).
Cells which have temporarily ceased to grow may become rich in
storage products, both nitrogenous and carbohydrate. Both carrot and
potato cells are rich in alcohol-soluble nitrogen compounds, and these
contain much free amino acid and also amide nitrogen in the form of
asparagine and glutamine. A speedy consequence of the growth stimulus
is the rapid conversion of these soluble nitrogen compounds into protein;
this is also true in cells of tumours (Neish and Hibbert, 1943). As this
recrudescence of protein synthesis occurs in dividing cells, the relative
composition of the residual soluble nitrogen in the tissue, which falls
to a very low level, changes quite markedly. Whereas asparagine is
conspicuous in the resting cells, it disappears almost entirely from the
growing cells, while glutamine tends to persist in the growing system.
Thus the balance shifts from a high soluble nitrogen reserve in which
asparagine may be prominent to a low soluble nitrogen reserve in which
alanine and glutamine tend to be prominent and, as these soluble
reserves are continually depleted by growth, they are replenished either
from the casein hydrolysate supplied with the external medium, or by
direct synthesis from nitrate nitrogen (Steward and Pollard, 1957;
Steward, Thompson and Pollard, 1958).
Meanwhile, however, the total protein has markedly increased in the
cells stimulated into growth; but, of greater significance than the mere
increment of total protein, is a change in its over-all composition. This
was first recognized by the presence of hydroxyproline, in amounts far
higher than expected, in the hydrolysate of the alcohol insoluble protein
of tissues (carrot root and potato tuber) grown after treatment with
coconut milk. Moreover, other rapidly proliferating cell systems such
as the spontaneous tumours on certain hybrids of tobacco, or the
cells stimulated into rapid growth by the effect of the crown gall organism on Kalanchoe or Bryophyllum (Steward et al., 1955), and even
callus from leaf tissues of Agave tourney ana (Weinstein et al., 1959) were