10. ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION
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pea tissue, and approximately 60% reversed in the light in the case of
Avena sections. Since the reversal of CO inhibition by light is considered
specific for cytochrome oxidase, there is little doubt that the energy for
growth in these experiments was provided in an oxidation pathway
mediated by cytochrome oxidase. Similarly, the inhibition by CO of
water uptake in potato slices (229) was light reversible, as was the COinhibited salt absorption of barley roots (230). In these few cases, therefore, there is sufficient evidence to implicate the cytochrome respiratory
chain as the source of energy for these forms of work. The relationships
between respiration and cellular work have been discussed by Laties in
a recent, valuable paper (231).
If alternate pathways for oxidation do exist in some organisms, the
question of the physiological significance of these oxidation pathways is
important. Although a major role should be that of conservation of
energy in the form of energy-rich compounds, there is little direct evidence to indicate that energy can be made available other than through
oxidations mediated by cytochrome oxidase. Instead, the non-cytochrome
respiratory chains may constitute a mechanism for dissipating reducing
power represented by DPNH or TPNH and accumulated during some
special activity of the organism. Or the chain may function to keep
easily oxidized substances as glutathione, ascorbic acid, and phenols in
the reduced state.
Many enzymes in addition to cytochrome oxidase catalyze reactions
involving molecular oxygen. These enzymes have been classified and
discussed in some detail (232). However, only a few of these catalysts
can seriously be considered for a role in cellular respiration. These are
the enzymes which directly or indirectly can participate in the oxidation
of reduced pyridine nucleotides, flavoproteins, or cytochromes. Some of
these are discussed briefly.
A. ASCORBIC ACID OXIDASE
The widespread distribution of ascorbic acid oxidase in higher plants
was a major factor in assigning a tentative role in respiration to this
enzyme. The enzyme has been extensively purified (233) and characterized, and has been implicated in the respiration of germinating wheat
seedlings (99) and barley roots (227, 234). On the basis of the low
affinity of the enzyme for 0 2 , however, Bonner has concluded that
ascorbic acid oxidase cannot play a significant role in any tissues except
those which are highly aerobic in nature (224).
In contrast to this, Mapson and Moustafa have stated that ascorbic
acid and glutathione can function in a respiratory system which may
account for as much as 25% of the respiration of pea cotyledons (235).
473
pea tissue, and approximately 60% reversed in the light in the case of
Avena sections. Since the reversal of CO inhibition by light is considered
specific for cytochrome oxidase, there is little doubt that the energy for
growth in these experiments was provided in an oxidation pathway
mediated by cytochrome oxidase. Similarly, the inhibition by CO of
water uptake in potato slices (229) was light reversible, as was the COinhibited salt absorption of barley roots (230). In these few cases, therefore, there is sufficient evidence to implicate the cytochrome respiratory
chain as the source of energy for these forms of work. The relationships
between respiration and cellular work have been discussed by Laties in
a recent, valuable paper (231).
If alternate pathways for oxidation do exist in some organisms, the
question of the physiological significance of these oxidation pathways is
important. Although a major role should be that of conservation of
energy in the form of energy-rich compounds, there is little direct evidence to indicate that energy can be made available other than through
oxidations mediated by cytochrome oxidase. Instead, the non-cytochrome
respiratory chains may constitute a mechanism for dissipating reducing
power represented by DPNH or TPNH and accumulated during some
special activity of the organism. Or the chain may function to keep
easily oxidized substances as glutathione, ascorbic acid, and phenols in
the reduced state.
Many enzymes in addition to cytochrome oxidase catalyze reactions
involving molecular oxygen. These enzymes have been classified and
discussed in some detail (232). However, only a few of these catalysts
can seriously be considered for a role in cellular respiration. These are
the enzymes which directly or indirectly can participate in the oxidation
of reduced pyridine nucleotides, flavoproteins, or cytochromes. Some of
these are discussed briefly.
A. ASCORBIC ACID OXIDASE
The widespread distribution of ascorbic acid oxidase in higher plants
was a major factor in assigning a tentative role in respiration to this
enzyme. The enzyme has been extensively purified (233) and characterized, and has been implicated in the respiration of germinating wheat
seedlings (99) and barley roots (227, 234). On the basis of the low
affinity of the enzyme for 0 2 , however, Bonner has concluded that
ascorbic acid oxidase cannot play a significant role in any tissues except
those which are highly aerobic in nature (224).
In contrast to this, Mapson and Moustafa have stated that ascorbic
acid and glutathione can function in a respiratory system which may
account for as much as 25% of the respiration of pea cotyledons (235).