284
GEORGE G. LATIES
oxidase-mediated oxidations of the tricarboxylic acid cycle, the evidence
favors this view with respect to the higher plants.
In spite of the preeminent role just ascribed to that part of the respiration which proceeds through the tricarboxylic acid cycle and the cytochrome oxidase system, an interesting variety of alternative respiratory
pathways has been described in addition to the few mentioned above.
In many instances it is not sure that the respiratory sequences which
may be constructed in vitro actually occur within the cell. Assuming
they do occur in vivo, their physiological significance still remains an
open question. For example, Wosilait et al. (1954) have described a
quinone reductase in peas which is capable of causing the oxidation of
either DPNH or TPNH, and which may be linked to polyphenolase and
thereby to the utilization of molecular oxygen. Inexplicably, very low
levels of dinitrophenol almost completely inhibit the quinone reductase.
Zelitch (1953) and Zelitch and Ochoa (1953) have demonstrated a system which may oxidize both DPNH and TPNH by molecular oxygen
through the combined activity of glycolic acid oxidase and glyoxylic acid
reductase, the latter oxidizing the reduced coenzymes directly. The
cyanide-insensitive glycolic acid oxidase was shown to be a flavoprotein.
The activity of this enzyme in green tissues far exceeds that in etiolated
tissues (Noll and Burris, 1954).
In a variety of oxidative systems ascorbic acid oxidase plays a part.
Systems which oxidize hexose diphosphate (in barley) or malate (in
wheat), and which may be linked to ascorbic acid oxidase, have been
reconstructed by James et al. (1944) and by Waygood (1950), respectively. Beevers (1954) has isolated a DPN-specific system from cucumbers in which the oxidation of DPNH is effected via ascorbate and ascorbic acid oxidase. It is of particular interest that dehydroascorbic acid
fails to act as a hydrogen carrier in this system, although ascorbic acid
does so. Nason et al. (1954) have described a similar situation in peas,
and have suggested that the intermediate electron acceptor arising from
the oxidation of ascorbic acid is possibly a free radical such as monodehydroascorbic acid. They point out that only oxidizing catalysts undergoing
single electron changes (Cu
+ + , Fe
+ + + ) can substitute for ascorbic acid
oxidase in the respiratory chain. With yet another enzyme from the pea,
Mathews (1951) has demonstrated the oxidation of DPNH in a system
where ascorbic acid serves a catalytic function, dehydroascorbic acid
does not, where ascorbic acid oxidase is not implicated, and the oxidation is cyanide-resistant.
Glutathione has been implicated as an electron carrier in the oxidation of one or the other pyridine nucleotide in a variety of plant tissue
extracts (Conn and Vennesland, 1951; Anderson et al., 1952; Mapson and
GEORGE G. LATIES
oxidase-mediated oxidations of the tricarboxylic acid cycle, the evidence
favors this view with respect to the higher plants.
In spite of the preeminent role just ascribed to that part of the respiration which proceeds through the tricarboxylic acid cycle and the cytochrome oxidase system, an interesting variety of alternative respiratory
pathways has been described in addition to the few mentioned above.
In many instances it is not sure that the respiratory sequences which
may be constructed in vitro actually occur within the cell. Assuming
they do occur in vivo, their physiological significance still remains an
open question. For example, Wosilait et al. (1954) have described a
quinone reductase in peas which is capable of causing the oxidation of
either DPNH or TPNH, and which may be linked to polyphenolase and
thereby to the utilization of molecular oxygen. Inexplicably, very low
levels of dinitrophenol almost completely inhibit the quinone reductase.
Zelitch (1953) and Zelitch and Ochoa (1953) have demonstrated a system which may oxidize both DPNH and TPNH by molecular oxygen
through the combined activity of glycolic acid oxidase and glyoxylic acid
reductase, the latter oxidizing the reduced coenzymes directly. The
cyanide-insensitive glycolic acid oxidase was shown to be a flavoprotein.
The activity of this enzyme in green tissues far exceeds that in etiolated
tissues (Noll and Burris, 1954).
In a variety of oxidative systems ascorbic acid oxidase plays a part.
Systems which oxidize hexose diphosphate (in barley) or malate (in
wheat), and which may be linked to ascorbic acid oxidase, have been
reconstructed by James et al. (1944) and by Waygood (1950), respectively. Beevers (1954) has isolated a DPN-specific system from cucumbers in which the oxidation of DPNH is effected via ascorbate and ascorbic acid oxidase. It is of particular interest that dehydroascorbic acid
fails to act as a hydrogen carrier in this system, although ascorbic acid
does so. Nason et al. (1954) have described a similar situation in peas,
and have suggested that the intermediate electron acceptor arising from
the oxidation of ascorbic acid is possibly a free radical such as monodehydroascorbic acid. They point out that only oxidizing catalysts undergoing
single electron changes (Cu
+ + , Fe
+ + + ) can substitute for ascorbic acid
oxidase in the respiratory chain. With yet another enzyme from the pea,
Mathews (1951) has demonstrated the oxidation of DPNH in a system
where ascorbic acid serves a catalytic function, dehydroascorbic acid
does not, where ascorbic acid oxidase is not implicated, and the oxidation is cyanide-resistant.
Glutathione has been implicated as an electron carrier in the oxidation of one or the other pyridine nucleotide in a variety of plant tissue
extracts (Conn and Vennesland, 1951; Anderson et al., 1952; Mapson and