RESPIRATION RATE IN PLANTS
225
lating ATPase activity, since isotope dilution would then be expected as
a consequence of ATP hydrolysis. The suggestion of Cohn that dinitrophenol exerts its effect before inorganic phosphate takes part in oxidative phosphorylation may be reconciled with the observations that
dinitrophenol stimulates ATPase activity if it be considered, as Lardy
and Wellman (1953) have suggested, that the so-called ATPase activity
of carefully prepared cellular particles depends on the partial reversal
of the phosphorylative process, as shown in Fig. 4. That such reversal
can occur has been amply demonstrated by Boyer et al. (1954) and by
Harrison, Boyer, and Falcone (1955). The suggestion is included in
Fig. 4 that dinitrophenol can catalyze the hydrolysis of both types of
high-energy bonds depicted in the figure. The hydrolysis of the first type
of linkage naturally prevents the formation of the second type in a system
where they occur sequentially.
AU ATPase activity does not depend upon a partial reversal of the
phosphorylative process. Enzymes can be prepared which remove phosphate from ATP irreversibly. However, it appears that in general ATPase
has little to do with rate regulation in vivo. Carefully prepared mitochondria show little or no ATPase activity, the activity being "latent" (Potter
et al, 1953). By the same token, such mitochondria show little oxidative
activity unless a phosphate-accepting system is supplied, or is generated
from ATP. Whatever the nature of the ATPase demonstrable in vitro, it
is questionable whether ATPase exists as such under natural conditions.
In summary, the available evidence indicates that the majority of the
oxidations within the tricarboxylic acid cycle, as well as the oxidation
of phosphoglyceraldehyde during glycolysis, are obligatorily coupled to
the phosphorylative process under natural conditions. As a consequence,
the respiratory rate is governed primarily by the level of phosphate acceptor, the latter being determined in turn by the prevalence and activity
of transphosphorylating systems, as well as by the rate of utilization of
phosphorylated acceptors in cellular reactions. Dinitrophenol, in uncoupling the phosphorylative from the oxidative systems, removes the
requirement for acceptors, and probably for inorganic phosphate as well.
II. REGULATION OF RESPIRATION RATE IN PLANT TISSUES
If an oxidative unit is thought to consist of a substrate dehydrogenase
together with a series of electron carriers ending with the terminal oxidase, the contribution to the total respiration by a given type of oxidative
unit will depend upon the number of such units in operation and the
rate at which electrons are passed through the unit. In the simplest
view, the fraction of the available units in operation will be determined
by the concentration of the specific substrate. In a phosphorylative sys-
225
lating ATPase activity, since isotope dilution would then be expected as
a consequence of ATP hydrolysis. The suggestion of Cohn that dinitrophenol exerts its effect before inorganic phosphate takes part in oxidative phosphorylation may be reconciled with the observations that
dinitrophenol stimulates ATPase activity if it be considered, as Lardy
and Wellman (1953) have suggested, that the so-called ATPase activity
of carefully prepared cellular particles depends on the partial reversal
of the phosphorylative process, as shown in Fig. 4. That such reversal
can occur has been amply demonstrated by Boyer et al. (1954) and by
Harrison, Boyer, and Falcone (1955). The suggestion is included in
Fig. 4 that dinitrophenol can catalyze the hydrolysis of both types of
high-energy bonds depicted in the figure. The hydrolysis of the first type
of linkage naturally prevents the formation of the second type in a system
where they occur sequentially.
AU ATPase activity does not depend upon a partial reversal of the
phosphorylative process. Enzymes can be prepared which remove phosphate from ATP irreversibly. However, it appears that in general ATPase
has little to do with rate regulation in vivo. Carefully prepared mitochondria show little or no ATPase activity, the activity being "latent" (Potter
et al, 1953). By the same token, such mitochondria show little oxidative
activity unless a phosphate-accepting system is supplied, or is generated
from ATP. Whatever the nature of the ATPase demonstrable in vitro, it
is questionable whether ATPase exists as such under natural conditions.
In summary, the available evidence indicates that the majority of the
oxidations within the tricarboxylic acid cycle, as well as the oxidation
of phosphoglyceraldehyde during glycolysis, are obligatorily coupled to
the phosphorylative process under natural conditions. As a consequence,
the respiratory rate is governed primarily by the level of phosphate acceptor, the latter being determined in turn by the prevalence and activity
of transphosphorylating systems, as well as by the rate of utilization of
phosphorylated acceptors in cellular reactions. Dinitrophenol, in uncoupling the phosphorylative from the oxidative systems, removes the
requirement for acceptors, and probably for inorganic phosphate as well.
II. REGULATION OF RESPIRATION RATE IN PLANT TISSUES
If an oxidative unit is thought to consist of a substrate dehydrogenase
together with a series of electron carriers ending with the terminal oxidase, the contribution to the total respiration by a given type of oxidative
unit will depend upon the number of such units in operation and the
rate at which electrons are passed through the unit. In the simplest
view, the fraction of the available units in operation will be determined
by the concentration of the specific substrate. In a phosphorylative sys-