RESPIRATION RATE IN PLANTS
235
when the reasons for aerobic alcohol production are understood (see
below). Beevers (1953) caused a variety of higher plant tissues to ferment in air by administering dinitrophenol. Newcomb (1950) evoked
aerobic alcohol production in tobacco callus by the same means.
When it is considered that perhaps thirty-six high-energy phosphate
bonds may be produced per molecule of hexose oxidized, in comparison
to two per molecule fermented, it is easy to see how the phosphate acceptor system may become rapidly saturated by the products of aerobic
phosphorylation. The single glycolytic step which is oxidative, namely,
the oxidation of phosphoglyceraldehyde, is also phosphorylative, and
therefore ultimately dependent upon the presence of sufficient phosphate
acceptor. Since glycolytic phosphorylation is insensitive to dinitrophenol
(Clowes and Keltch, 1951), the rate at which glycolysis proceeds in the
presence of dinitrophenol remains dependent upon the concentration
of phosphate acceptors. In the presence of uncoupling agents the phosphate acceptors which are normally preempted by the compulsorily aerobic electron-chain phosphorylations become totally available for the glycolytic process, which consequently proceeds more rapidly. Glycolytic
phosphorylation, being resistant to uncoupling, provides enough ATP
to bring about the initial phosphorylation of the hexose molecules necessary for the glycolytic process itself, although this requirement is frequently met in experiments by offering hexose diphosphate as the initial
substrate. Thus in general the Pasteur effect could be ascribed to a limitation both of phosphate acceptor and inorganic phosphate. However,
Terner (1954) has pointed out that the inorganic phosphate concentration at the end of his experiments with mammary gland homogenates
was relatively high, and he therefore ascribed the Pasteur effect entirely
to the unavailability of phosphate acceptor. It seems likely that the same
situation obtains in most plant tissues. Phosphorylative studies with
pea seeds (Rowan et al., 1956) have reaffirmed this hypothesis.
An alternative mechanism proposed for the Pasteur effect (Gottschalk,
1941) explains the lowering of glycolysis under aerobic conditions by
suggesting that the numerous coenzyme-linked dehydrogenations of the
aerobic respiratory system compete with glyceraldehyde phosphate dehydrogenase for available pyridine nucleotide coenzymes (DPN, TPN).
In this way the coenzymes are kept predominantly in the reduced state,
thereby limiting the oxidative step of glycolysis. An essential part of this
hypothesis is that in the steady state the ratio of DPN/DPNH must be
relatively low. This presumption seems reasonable, since the stimulation
of aerobic respiration by uncoupling agents suggests that it is not the reduction of the coenzymes which is rate limiting, but rather one or more
steps in the electron chain.
235
when the reasons for aerobic alcohol production are understood (see
below). Beevers (1953) caused a variety of higher plant tissues to ferment in air by administering dinitrophenol. Newcomb (1950) evoked
aerobic alcohol production in tobacco callus by the same means.
When it is considered that perhaps thirty-six high-energy phosphate
bonds may be produced per molecule of hexose oxidized, in comparison
to two per molecule fermented, it is easy to see how the phosphate acceptor system may become rapidly saturated by the products of aerobic
phosphorylation. The single glycolytic step which is oxidative, namely,
the oxidation of phosphoglyceraldehyde, is also phosphorylative, and
therefore ultimately dependent upon the presence of sufficient phosphate
acceptor. Since glycolytic phosphorylation is insensitive to dinitrophenol
(Clowes and Keltch, 1951), the rate at which glycolysis proceeds in the
presence of dinitrophenol remains dependent upon the concentration
of phosphate acceptors. In the presence of uncoupling agents the phosphate acceptors which are normally preempted by the compulsorily aerobic electron-chain phosphorylations become totally available for the glycolytic process, which consequently proceeds more rapidly. Glycolytic
phosphorylation, being resistant to uncoupling, provides enough ATP
to bring about the initial phosphorylation of the hexose molecules necessary for the glycolytic process itself, although this requirement is frequently met in experiments by offering hexose diphosphate as the initial
substrate. Thus in general the Pasteur effect could be ascribed to a limitation both of phosphate acceptor and inorganic phosphate. However,
Terner (1954) has pointed out that the inorganic phosphate concentration at the end of his experiments with mammary gland homogenates
was relatively high, and he therefore ascribed the Pasteur effect entirely
to the unavailability of phosphate acceptor. It seems likely that the same
situation obtains in most plant tissues. Phosphorylative studies with
pea seeds (Rowan et al., 1956) have reaffirmed this hypothesis.
An alternative mechanism proposed for the Pasteur effect (Gottschalk,
1941) explains the lowering of glycolysis under aerobic conditions by
suggesting that the numerous coenzyme-linked dehydrogenations of the
aerobic respiratory system compete with glyceraldehyde phosphate dehydrogenase for available pyridine nucleotide coenzymes (DPN, TPN).
In this way the coenzymes are kept predominantly in the reduced state,
thereby limiting the oxidative step of glycolysis. An essential part of this
hypothesis is that in the steady state the ratio of DPN/DPNH must be
relatively low. This presumption seems reasonable, since the stimulation
of aerobic respiration by uncoupling agents suggests that it is not the reduction of the coenzymes which is rate limiting, but rather one or more
steps in the electron chain.