RESPIRATION RATE IN PLANTS
273
The system which Lehninger describes resembles the microsomal
DPNH oxidase reported by Chance and Williams, which is nonphosphorylative, bypasses cytochrome b, and terminates in the cytochromecytochrome oxidase system (see Martin and Morton, 1956). Numerous
DPNH-cytochrome c reductases have been described to date, none of
which bear a resemblance to cytochrome b. Thus, it is not difficult to
envisage the operation of cytochrome oxidase in the absence of cytochrome b-mediated respiration. (See Lundegárdh, 1955b.) It would
be interesting to know whether the suppression of the respiration in
which cytochrome b is implicated results in a greater contribution by
the nonphosphorylative pathway.
Lundegárdh substantiates the observations of Robertson et al. (1951)
that dinitrophenol prevents salt accumulation, and now imputes the
effectiveness of dinitrophenol to the uncoupling of phosphorylations
which specifically accompany the oxidation of cytochrome b. It has
already been indicated that dinitrophenol abolishes all phosphorylation
associated with electron transport accompanying the tricarboxylic acid
cycle (see Hunter, 1951). Although Lundegárdh now considers that an
anion carrier, dependent upon ATP for its formation, transports anions
through the permeability barrier, he maintains that anions are delivered
to the carrier through the previously described cytochrome electron
ladder. Thus the coenzymatic function ascribed to anions is still envisaged. However, anions could exert the postulated coenzymatic function without any necessity for directional transport. Conceivably an
effect of salt on the respiration, independent of the absorption process,
might be sought here. The stimulation of cytochrome oxidase by neutral salts in vitro (Riley, 1950; Lundegárdh, 1953b) is effected by salt
concentrations in excess of 0.1 Μ whereas salt respiration is evoked by
salt concentrations of approximately 0.001 M. The high-salt and
low-salt responses may conceivably be quite different, and it remains
possible that the low salt requirement has in the past been met
by salt in the tissue extracts, or by buffer salts during the enzymatic
measurements. The observation most difficult to reconcile with the notion that the activity of the cytochrome system depends upon the concomitant transport of salt is the pronounced stimulation of cytochromeoxidase-mediated respiration by dinitrophenol (Robertson et al., 1951).
Since the cytochrome oxidase activity can be increased several fold in
the absence of salt, and under conditions, furthermore, where salt accumulation into the vacuole cannot take place, the movement of electrons through the cytochrome system clearly cannot depend upon the
movement of anions in the opposite direction.
In addition to the effect of dinitrophenol on salt accumulation, other
273
The system which Lehninger describes resembles the microsomal
DPNH oxidase reported by Chance and Williams, which is nonphosphorylative, bypasses cytochrome b, and terminates in the cytochromecytochrome oxidase system (see Martin and Morton, 1956). Numerous
DPNH-cytochrome c reductases have been described to date, none of
which bear a resemblance to cytochrome b. Thus, it is not difficult to
envisage the operation of cytochrome oxidase in the absence of cytochrome b-mediated respiration. (See Lundegárdh, 1955b.) It would
be interesting to know whether the suppression of the respiration in
which cytochrome b is implicated results in a greater contribution by
the nonphosphorylative pathway.
Lundegárdh substantiates the observations of Robertson et al. (1951)
that dinitrophenol prevents salt accumulation, and now imputes the
effectiveness of dinitrophenol to the uncoupling of phosphorylations
which specifically accompany the oxidation of cytochrome b. It has
already been indicated that dinitrophenol abolishes all phosphorylation
associated with electron transport accompanying the tricarboxylic acid
cycle (see Hunter, 1951). Although Lundegárdh now considers that an
anion carrier, dependent upon ATP for its formation, transports anions
through the permeability barrier, he maintains that anions are delivered
to the carrier through the previously described cytochrome electron
ladder. Thus the coenzymatic function ascribed to anions is still envisaged. However, anions could exert the postulated coenzymatic function without any necessity for directional transport. Conceivably an
effect of salt on the respiration, independent of the absorption process,
might be sought here. The stimulation of cytochrome oxidase by neutral salts in vitro (Riley, 1950; Lundegárdh, 1953b) is effected by salt
concentrations in excess of 0.1 Μ whereas salt respiration is evoked by
salt concentrations of approximately 0.001 M. The high-salt and
low-salt responses may conceivably be quite different, and it remains
possible that the low salt requirement has in the past been met
by salt in the tissue extracts, or by buffer salts during the enzymatic
measurements. The observation most difficult to reconcile with the notion that the activity of the cytochrome system depends upon the concomitant transport of salt is the pronounced stimulation of cytochromeoxidase-mediated respiration by dinitrophenol (Robertson et al., 1951).
Since the cytochrome oxidase activity can be increased several fold in
the absence of salt, and under conditions, furthermore, where salt accumulation into the vacuole cannot take place, the movement of electrons through the cytochrome system clearly cannot depend upon the
movement of anions in the opposite direction.
In addition to the effect of dinitrophenol on salt accumulation, other