218
F. C. STEWARD AND Η. Y. MOHAN
RAM
found to contain protein unexpectedly rich in hydroxy proline. Thus,
in all the above cases, in which cells had entered upon a stage of rapid
and rather random proliferation, there is the very suggestive presence
of a protein moiety which is rich in hydroxyproline, but which was
not previously prominent in the quiescent cells. Work with carrot
tissue explants has shown that the hydroxyproline which is contained
in the protein arose after the protein was synthesized and after it
had embodied intact proline molecules from the soluble phases of the
cell (Steward and Pollard, 1958; Pollard and Steward, 1959). Moreover,
the protein moiety so formed proves to be remarkably resistant to
metabolic usage or 'turnover', for any
14
C-labelled proline which is
supplied and enters the protein, as such, is not re-used and is only
subject to partial conversion to hydroxyproline.
It is, therefore, extremely tempting to see in the presence of this
unusual protein moiety some essential part of the cellular machinery
which permits these cells to divide rapidly. As indicated below, the
thought—as yet unproven—is that this protein may accumulate in
fibrous elements of the cytoplasm, probably even in the tube-like canals
of the reticulum itself, or at least in the very coarse strands which
abound in these highly vacuolated but growing cells (cf. Steward and
Pollard, 1959, p. 205).
Even before radioactively labelled substrates were applied to cells
which had been stimulated to grow by chemical means, a concept had
emerged which suggested that it is only in resting cells that one can
easily explain the course of respiration in terms of the flow of carbon
from carbohydrates in the form of a temporary storage product (i.e.
starch or sugar) through the events of glycolysis and into the aerobic
respiratory cycle usually known as the organic acid cycle. This may well
be true of the cell which is, as it were, 'ticking over' and allowing much
of its energy to run to waste. But the respiratory metabolism of cells
in a state of more active metabolism, protein synthesis and growth,
requires different concepts (Steward, Bidwell and Yemm, 1956, 1958;
Steward and Bidwell, 1958; also see Steward and Pollard, 1959). When
the metabolic machine is geared toward growth and protein synthesis,
it seems that there is superimposed upon the resting pattern of metabolism, a different and more active one. The present interpretation of
this superimposed respiratory pattern which is characteristic of the
actively growing and metabolizing cell is as follows.
It is now conceived that protein is made in at least two distinct ways.
In the one, certain preformed amino acids, which exist in quantity as
such in the cells, may be directly incorporated into the protein molecule.
But in the other, the carbon of the free amino acids of the cell seems to
be more remote from the site of protein synthesis than the carbon of
F. C. STEWARD AND Η. Y. MOHAN
RAM
found to contain protein unexpectedly rich in hydroxy proline. Thus,
in all the above cases, in which cells had entered upon a stage of rapid
and rather random proliferation, there is the very suggestive presence
of a protein moiety which is rich in hydroxyproline, but which was
not previously prominent in the quiescent cells. Work with carrot
tissue explants has shown that the hydroxyproline which is contained
in the protein arose after the protein was synthesized and after it
had embodied intact proline molecules from the soluble phases of the
cell (Steward and Pollard, 1958; Pollard and Steward, 1959). Moreover,
the protein moiety so formed proves to be remarkably resistant to
metabolic usage or 'turnover', for any
14
C-labelled proline which is
supplied and enters the protein, as such, is not re-used and is only
subject to partial conversion to hydroxyproline.
It is, therefore, extremely tempting to see in the presence of this
unusual protein moiety some essential part of the cellular machinery
which permits these cells to divide rapidly. As indicated below, the
thought—as yet unproven—is that this protein may accumulate in
fibrous elements of the cytoplasm, probably even in the tube-like canals
of the reticulum itself, or at least in the very coarse strands which
abound in these highly vacuolated but growing cells (cf. Steward and
Pollard, 1959, p. 205).
Even before radioactively labelled substrates were applied to cells
which had been stimulated to grow by chemical means, a concept had
emerged which suggested that it is only in resting cells that one can
easily explain the course of respiration in terms of the flow of carbon
from carbohydrates in the form of a temporary storage product (i.e.
starch or sugar) through the events of glycolysis and into the aerobic
respiratory cycle usually known as the organic acid cycle. This may well
be true of the cell which is, as it were, 'ticking over' and allowing much
of its energy to run to waste. But the respiratory metabolism of cells
in a state of more active metabolism, protein synthesis and growth,
requires different concepts (Steward, Bidwell and Yemm, 1956, 1958;
Steward and Bidwell, 1958; also see Steward and Pollard, 1959). When
the metabolic machine is geared toward growth and protein synthesis,
it seems that there is superimposed upon the resting pattern of metabolism, a different and more active one. The present interpretation of
this superimposed respiratory pattern which is characteristic of the
actively growing and metabolizing cell is as follows.
It is now conceived that protein is made in at least two distinct ways.
In the one, certain preformed amino acids, which exist in quantity as
such in the cells, may be directly incorporated into the protein molecule.
But in the other, the carbon of the free amino acids of the cell seems to
be more remote from the site of protein synthesis than the carbon of