280
GEORGE G. LATIES
are associated with ripening. When slices of unripe avocado were treated
with dinitrophenol for varying periods of time, they rapidly softened,
and chemical changes which normally accompany ripening were demonstrated (Millerd and Bonner, 1953). If the advent of the climacteric is
to be attributed to natural uncoupling, it must be recognized that the
uncoupling phenomenon is in all probability not complete, nor can it be
invoked as the sole explanation for ripening. For as shown by Biale
(1946), both the climacteric and the ripening process in avocado fruits
are delayed by lowering the p 0 . 2 , a condition which would tend to simulate uncoupling in so far as the phosphorylative metabolism would be
minimized. Furthermore, apple slices frequently respond somewhat to
dinitrophenol at the height of the climacteric, and at the same time still
exhibit an ability to synthesize protein (Pearson and Robertson, 1953;
Hulme, 1954).
In a further attempt to understand the respiratory changes associated
with fruit ripening. Romani and Biale (in press) examined the effect of
dinitrophenol on the oxidation of a-ketoglutarate, pyruvate, succinate,
and malate by mitochondria from avocado fruit in different stages of
development. Their data are provocative but difficult to interpret. In
general, with all substrates, they obtained a P/O ratio which was higher
with mitochondria prepared from freshly picked fruit, or fruit at the preclimacteric minimum. More remarkable, the effectiveness of dinitrophenol in uncoupling phosphorylation was in all cases great with particles from fresh and preclimacteric material, and slight with particles from
climacteric material. With malate as substrate, dinitrophenol had practically no effect upon particles from climacteric fruit. These findings,
however, cannot be compared directly to those of Millerd et al., since in
the experiments of the latter the effect of dinitrophenol was tested only
with particles from preclimacteric fruit. It is still not ruled out that
an endogenous uncoupling agent plays an important role in the development of the climacteric, for as has been pointed out, the cytoplasmic
uncoupling factor in avocado fruit is nonparticulate and hence is not evident in mitochondrial preparations. Nevertheless, the observations of
Romani and Biale pose a formidable question with respect to the failure
of dinitrophenol to inhibit phosphorylation more effectively. In some
plant mitochondrial preparations, the enzymes which naturally transfer phosphate from the site of initial phosphorylation may stimulate
respiration more effectively than does dinitrophenol (Laties, 1953a).
There remains therefore some possibility that mitochondria from climacteric avocado fruits have a much better developed phosphate-transferring
system than do mitochondria from preclimacteric fruit, and that this
transferring system is more effective in expediting respiration than is the
GEORGE G. LATIES
are associated with ripening. When slices of unripe avocado were treated
with dinitrophenol for varying periods of time, they rapidly softened,
and chemical changes which normally accompany ripening were demonstrated (Millerd and Bonner, 1953). If the advent of the climacteric is
to be attributed to natural uncoupling, it must be recognized that the
uncoupling phenomenon is in all probability not complete, nor can it be
invoked as the sole explanation for ripening. For as shown by Biale
(1946), both the climacteric and the ripening process in avocado fruits
are delayed by lowering the p 0 . 2 , a condition which would tend to simulate uncoupling in so far as the phosphorylative metabolism would be
minimized. Furthermore, apple slices frequently respond somewhat to
dinitrophenol at the height of the climacteric, and at the same time still
exhibit an ability to synthesize protein (Pearson and Robertson, 1953;
Hulme, 1954).
In a further attempt to understand the respiratory changes associated
with fruit ripening. Romani and Biale (in press) examined the effect of
dinitrophenol on the oxidation of a-ketoglutarate, pyruvate, succinate,
and malate by mitochondria from avocado fruit in different stages of
development. Their data are provocative but difficult to interpret. In
general, with all substrates, they obtained a P/O ratio which was higher
with mitochondria prepared from freshly picked fruit, or fruit at the preclimacteric minimum. More remarkable, the effectiveness of dinitrophenol in uncoupling phosphorylation was in all cases great with particles from fresh and preclimacteric material, and slight with particles from
climacteric material. With malate as substrate, dinitrophenol had practically no effect upon particles from climacteric fruit. These findings,
however, cannot be compared directly to those of Millerd et al., since in
the experiments of the latter the effect of dinitrophenol was tested only
with particles from preclimacteric fruit. It is still not ruled out that
an endogenous uncoupling agent plays an important role in the development of the climacteric, for as has been pointed out, the cytoplasmic
uncoupling factor in avocado fruit is nonparticulate and hence is not evident in mitochondrial preparations. Nevertheless, the observations of
Romani and Biale pose a formidable question with respect to the failure
of dinitrophenol to inhibit phosphorylation more effectively. In some
plant mitochondrial preparations, the enzymes which naturally transfer phosphate from the site of initial phosphorylation may stimulate
respiration more effectively than does dinitrophenol (Laties, 1953a).
There remains therefore some possibility that mitochondria from climacteric avocado fruits have a much better developed phosphate-transferring
system than do mitochondria from preclimacteric fruit, and that this
transferring system is more effective in expediting respiration than is the