RESPIRATION RATE IN PLANTS
233
dominantly in limiting oxidation to a single step of the cycle. By the
same token supposed reversals of malonate inhibition ought to be viewed
with caution, for the initial level of respiration might in fact be restored
by a single-step oxidation.
The respiration of cauliflower mitochondria may be increased by
dinitrophenol (Laties, 1953a). However, hexokinase, used as an enzymatic means of regenerating the phosphate acceptor, proved consistently to have more effect than dinitrophenol, and the suggestion has
been made that the efficiency of dinitrophenol as an uncoupler may vary
with mitochondrial preparations from different sources. The requirement for small quantities of adenylic acid as a prerequisite for the stimulation of respiration by dinitrophenol, as observed in the experiments of
Laties, may reasonably be explained on the basis that α-ketoglutarate
was used as the substrate. In view of the fact that the substrate-level
phosphorylation which accompanies α-ketoglutarate oxidation is resistant
to dinitrophenol and in the light of current knowledge regarding the
mechanism of this phosphorylation (see Fig. 2), it follows that adenylate
must be supplied as a phosphate acceptor for the substrate level oxidation
before dinitrophenol can stimulate oxidation within the respiratory chain.
Nevertheless, a dual role for adenylate, perhaps as a coenzyme (Romberg and Pricer, 1951; Kaplan et al., 1953), remains a possibility.
Cohn (1953) has shown that adenylic acid and Mg++ are necessary for
the exchange of O
18 of isotopically enriched inorganic phosphate with
the oxygen of water even though the exchange is clearly independent
of the formation of ATP.
As has already been pointed out, the control of respiratory rate in
higher plants is intimately related to the phosphorylative reactions which
accompany the respiratory oxidations. Such control is truly regulatory,
since the performance of chemical or physical work by the cell results
in a compensatory stimulation of the respiration. It must be keenly evident that the previous discussion of rate regulation has pertained exclusively to those oxidative pathways which normally result in the ultimate transfer of electrons through the "cytochrome-cytochrome oxidase"
system. Although cytochrome oxidase has been found in plant tissue
almost wherever it has been sought (Webster, 1952, 1954), there are
plentiful examples of the simultaneous presence of one or more additional terminal oxidases along with cytochrome oxidase. In addition to
polyphenolase and ascorbic oxidase, known oxidases in plant tissues now
include the cyanide-resistant glycolic oxidase (Zelitch and Ochoa, 1953),
oxalic oxidase (Finkle and Arnon, 1954), amine and aldehyde oxidases
(Kenten and Mann, 1951; Kenten, 1953), lipoxidase (Fritz and Beevers,
1955), and the oxidases of Arum (James and Beevers, 1950; Hackett,
233
dominantly in limiting oxidation to a single step of the cycle. By the
same token supposed reversals of malonate inhibition ought to be viewed
with caution, for the initial level of respiration might in fact be restored
by a single-step oxidation.
The respiration of cauliflower mitochondria may be increased by
dinitrophenol (Laties, 1953a). However, hexokinase, used as an enzymatic means of regenerating the phosphate acceptor, proved consistently to have more effect than dinitrophenol, and the suggestion has
been made that the efficiency of dinitrophenol as an uncoupler may vary
with mitochondrial preparations from different sources. The requirement for small quantities of adenylic acid as a prerequisite for the stimulation of respiration by dinitrophenol, as observed in the experiments of
Laties, may reasonably be explained on the basis that α-ketoglutarate
was used as the substrate. In view of the fact that the substrate-level
phosphorylation which accompanies α-ketoglutarate oxidation is resistant
to dinitrophenol and in the light of current knowledge regarding the
mechanism of this phosphorylation (see Fig. 2), it follows that adenylate
must be supplied as a phosphate acceptor for the substrate level oxidation
before dinitrophenol can stimulate oxidation within the respiratory chain.
Nevertheless, a dual role for adenylate, perhaps as a coenzyme (Romberg and Pricer, 1951; Kaplan et al., 1953), remains a possibility.
Cohn (1953) has shown that adenylic acid and Mg++ are necessary for
the exchange of O
18 of isotopically enriched inorganic phosphate with
the oxygen of water even though the exchange is clearly independent
of the formation of ATP.
As has already been pointed out, the control of respiratory rate in
higher plants is intimately related to the phosphorylative reactions which
accompany the respiratory oxidations. Such control is truly regulatory,
since the performance of chemical or physical work by the cell results
in a compensatory stimulation of the respiration. It must be keenly evident that the previous discussion of rate regulation has pertained exclusively to those oxidative pathways which normally result in the ultimate transfer of electrons through the "cytochrome-cytochrome oxidase"
system. Although cytochrome oxidase has been found in plant tissue
almost wherever it has been sought (Webster, 1952, 1954), there are
plentiful examples of the simultaneous presence of one or more additional terminal oxidases along with cytochrome oxidase. In addition to
polyphenolase and ascorbic oxidase, known oxidases in plant tissues now
include the cyanide-resistant glycolic oxidase (Zelitch and Ochoa, 1953),
oxalic oxidase (Finkle and Arnon, 1954), amine and aldehyde oxidases
(Kenten and Mann, 1951; Kenten, 1953), lipoxidase (Fritz and Beevers,
1955), and the oxidases of Arum (James and Beevers, 1950; Hackett,