Oscillatory Control of Glycolysis as Model for Timing Processes
209
phase, whereas pyruvate, when added at the NADH maximum brought
about a retardation. FRENKEL [8], working with beef heart extract, demonstrated a complete phase shift when adding AMP at minimum NADH.
The same chemical, however, caused no phase-shift when added at maximal
NADH.
Chemicals like adenosine nucleotides or pyruvate are involved not only
in glycolysis but in many other processes too. It is obvious that these
metabolic pathways should be able to control the glycolytic oscillator. This
biochemical oscillator is highly stable but there are chemical mechanisms for
setting and resetting it.
As already mentioned, an oscillating control system has to be able to
transfer its own rhythmicity to other metabolic pathways. In this connection PRESSMAN [9] made an instructive experiment. He was able to demonstrate that in rat liver mitochondria oscillations of NADH (under aerobic
conditions) are closely connected with potassium uptake, shrinking and
swelling of mitochondria and acidification of the medium. It may be that
some of these events participate in the control circuit producing oscillations,
but we can be sure that at least some of them are not the cause, but rather
the consequence of oscillatory control.
In yeast cells the glycolytic breakdown of glucose is not only an oscillatory process, but its interconversion to UDPG, an intermediate of glycogen
synthesis, is also a pulsed process. We cannot decide at present whether
glycolytic oscillations are responsible for these processes or whether pulsed
incorporation of glucose itself is the first cause of these rhythms.
In any case, however, there is ample evidence that a biochemical oscillator will transfer its rhythmicity to other metabolic events.
Analysis of the Oscillator Itself
Of course, fluctuating NADH levels indicate that glycolysis as a whole
will oscillate because NADH is produced as well as consumed in this metabolite sequence. The first question to be raised when NADH is oscillating
is: which other metabolites will do the same and what are their phase relations? To answer this, NADH was monitored by measuring fluorescence
in a batch of yeast cells, while samples were taken at different intervals as
shown by the dotted lines in fig. 3. These samples had to be deproteinized
and quantified for different metabolites. As the figure shows, the pool sizes
of intermediates also fluctuate. Their phases can differ very greatly from the
phase of NADH as demonstrated, for example, by G-6-P and F-6-P. So all
intermediates have to be checked. For establishing smaller differences in
phase, it is sometimes advisable to use the phase plan plot as proposed by
GHOSH and CHANCE [6]. The concentration of one chemical is plotted for
every sample against the concentration of another (fig. 4). NADH can be
14 3. Syrup. Quant. BioI.
209
phase, whereas pyruvate, when added at the NADH maximum brought
about a retardation. FRENKEL [8], working with beef heart extract, demonstrated a complete phase shift when adding AMP at minimum NADH.
The same chemical, however, caused no phase-shift when added at maximal
NADH.
Chemicals like adenosine nucleotides or pyruvate are involved not only
in glycolysis but in many other processes too. It is obvious that these
metabolic pathways should be able to control the glycolytic oscillator. This
biochemical oscillator is highly stable but there are chemical mechanisms for
setting and resetting it.
As already mentioned, an oscillating control system has to be able to
transfer its own rhythmicity to other metabolic pathways. In this connection PRESSMAN [9] made an instructive experiment. He was able to demonstrate that in rat liver mitochondria oscillations of NADH (under aerobic
conditions) are closely connected with potassium uptake, shrinking and
swelling of mitochondria and acidification of the medium. It may be that
some of these events participate in the control circuit producing oscillations,
but we can be sure that at least some of them are not the cause, but rather
the consequence of oscillatory control.
In yeast cells the glycolytic breakdown of glucose is not only an oscillatory process, but its interconversion to UDPG, an intermediate of glycogen
synthesis, is also a pulsed process. We cannot decide at present whether
glycolytic oscillations are responsible for these processes or whether pulsed
incorporation of glucose itself is the first cause of these rhythms.
In any case, however, there is ample evidence that a biochemical oscillator will transfer its rhythmicity to other metabolic events.
Analysis of the Oscillator Itself
Of course, fluctuating NADH levels indicate that glycolysis as a whole
will oscillate because NADH is produced as well as consumed in this metabolite sequence. The first question to be raised when NADH is oscillating
is: which other metabolites will do the same and what are their phase relations? To answer this, NADH was monitored by measuring fluorescence
in a batch of yeast cells, while samples were taken at different intervals as
shown by the dotted lines in fig. 3. These samples had to be deproteinized
and quantified for different metabolites. As the figure shows, the pool sizes
of intermediates also fluctuate. Their phases can differ very greatly from the
phase of NADH as demonstrated, for example, by G-6-P and F-6-P. So all
intermediates have to be checked. For establishing smaller differences in
phase, it is sometimes advisable to use the phase plan plot as proposed by
GHOSH and CHANCE [6]. The concentration of one chemical is plotted for
every sample against the concentration of another (fig. 4). NADH can be
14 3. Syrup. Quant. BioI.