10. ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION
477
In discussing the significance of the DPNH oxidase system of S.
faecalis Dolin points out that under aerobic conditions additional
sources of energy are available to the organism. Thus pyruvate, which is
reduced anaerobically, can be oxidized to acetyl-CoA. DPNH, which
reduces the pyruvate anaerobically in a non-phosphorylative reaction,
can be oxidized through the oxidase and peroxidase system described.
Since the growth of S. faecalis is increased 30 to 40% under aerobic conditions (124, 253), respiration in air can furnish energy for this increased
activity. However, oxidative phosphorylation by cell-free preparations
during aerobic oxidation and peroxidation has not been obtained.
Another flavoprotein enzyme which oxidizes the α-hydroxy acids,
glycolic and lactic, has been observed in higher plants (254). This enzyme, when coupled with the reduction of glyoxylic acid by a reductase
requiring either DPNH or TPNH, constitutes another pathway for oxidation of these coenzymes independent of the cytochromes (255). Although it has been suggested that the glycolic and glyoxylic enzymes
may play a role in respiration in green leaves, there is little supporting
evidence in work with intact tissues.
D. CYANIDE-INSENSITIVE RESPIRATION
The respiration of various plant tissues is insensitive to cyanide, carbon monoxide, and azide (227). Enzymes other than cytochrome oxidase have therefore been suggested as the terminal oxidase in these tissues. Some results obtained with the aroid spadix, which has been
studied extensively, will be described briefly.
James and Beevers (256) demonstrated that the respiration of tissue
slices from the spadix of Arum maculatum was not inhibited significantly by 10~
3 M HCN. However, the respiration of slices was dependent upon 0 2 partial pressure and was inhibited 75% when 0 2 concentration decreased from 21% to 5%. This dependence on 0 2 tension
and lack of inhibition by the classical inhibitors of cytochrome oxidase
prompted these workers to suggest that flavoproteins mediated the reduction of 0 2 in this tissue. Yocum and Hackett (257) in later studies
confirmed that respiration of spadices of Philodendron
grandifolium
was relatively insensitive to cyanide and carbon monoxide, but they
observed that the respiration rate was strongly dependent upon the
manner in which 0 2 was supplied to slices of this tissue. When the 0 2
was furnished to the slices contained in small respirometers in a moist
gas phase but with no suspending medium, the p0 2
50 (0 2 partial pressure for half maximal respiratory rate) was 0.002 atm. However, slices
in buffer exhibited a p0 2
50 of 0.16 atm. Thus a low affinity for 0 2 was
observed only when the 0 2 had to diffuse through the buffer to the
slices.
477
In discussing the significance of the DPNH oxidase system of S.
faecalis Dolin points out that under aerobic conditions additional
sources of energy are available to the organism. Thus pyruvate, which is
reduced anaerobically, can be oxidized to acetyl-CoA. DPNH, which
reduces the pyruvate anaerobically in a non-phosphorylative reaction,
can be oxidized through the oxidase and peroxidase system described.
Since the growth of S. faecalis is increased 30 to 40% under aerobic conditions (124, 253), respiration in air can furnish energy for this increased
activity. However, oxidative phosphorylation by cell-free preparations
during aerobic oxidation and peroxidation has not been obtained.
Another flavoprotein enzyme which oxidizes the α-hydroxy acids,
glycolic and lactic, has been observed in higher plants (254). This enzyme, when coupled with the reduction of glyoxylic acid by a reductase
requiring either DPNH or TPNH, constitutes another pathway for oxidation of these coenzymes independent of the cytochromes (255). Although it has been suggested that the glycolic and glyoxylic enzymes
may play a role in respiration in green leaves, there is little supporting
evidence in work with intact tissues.
D. CYANIDE-INSENSITIVE RESPIRATION
The respiration of various plant tissues is insensitive to cyanide, carbon monoxide, and azide (227). Enzymes other than cytochrome oxidase have therefore been suggested as the terminal oxidase in these tissues. Some results obtained with the aroid spadix, which has been
studied extensively, will be described briefly.
James and Beevers (256) demonstrated that the respiration of tissue
slices from the spadix of Arum maculatum was not inhibited significantly by 10~
3 M HCN. However, the respiration of slices was dependent upon 0 2 partial pressure and was inhibited 75% when 0 2 concentration decreased from 21% to 5%. This dependence on 0 2 tension
and lack of inhibition by the classical inhibitors of cytochrome oxidase
prompted these workers to suggest that flavoproteins mediated the reduction of 0 2 in this tissue. Yocum and Hackett (257) in later studies
confirmed that respiration of spadices of Philodendron
grandifolium
was relatively insensitive to cyanide and carbon monoxide, but they
observed that the respiration rate was strongly dependent upon the
manner in which 0 2 was supplied to slices of this tissue. When the 0 2
was furnished to the slices contained in small respirometers in a moist
gas phase but with no suspending medium, the p0 2
50 (0 2 partial pressure for half maximal respiratory rate) was 0.002 atm. However, slices
in buffer exhibited a p0 2
50 of 0.16 atm. Thus a low affinity for 0 2 was
observed only when the 0 2 had to diffuse through the buffer to the
slices.