466
VERNON H. CHELDELIN, CHIH H. WANG, AND TSOO E. KING
tissue ribose (in RNA). He concluded that the oxidative pentose cycle
could not account for the labeling observed. However, Horecker and
Mehler (219) recalculated the results on the assumption that the ribose
had arisen via reversal of the nonoxidative portion of the pentose cycle
and found excellent agreement with the expected values.
A third example, pointing to the importance of the cycle in a synthetic role, comprises the finding (32, 220) that the pentose cycle enzymes are more active and/or abundant during mammalian fetal development, and that in the final one-fourth to one-third of the gestation
period, the fetus develops "adult" characteristics with respect to carbohydrate breakdown, i.e., the glycolysis-Krebs cycle route is greatly preferred in the later stages as well as in postpartum life. These examples
support the idea that the pentose cycle may serve in many organisms
mainly as a pathway for synthesis, even though A. suboxydans (144)
and a few related microorganisms may under appropriate conditions
use this pathway oxidatively, for carbohydrate breakdown. The extent
to which oxidative metabolism of carbohydrates may be routed via the
pentose cycle will depend upon the simultaneous utilizability of other
pathways.
Finally, as suggested in the review by Horecker and Hiatt (2c), a
fourth possible role of the pentose cycle (here operating clockwise, or
in the forward direction) may be to supply TPNH to the system. In
many species, the pentose cycle is TPN-specific; glycolysis and Krebs
cycle oxidations are, on the other hand, largely DPN-requiring (a
possible exception in some systems is isocitric dehydrogenase) so that
processes such as fatty acid synthesis may derive significant amounts of
needed TPNH from phosphogluconate cleavage.
It can, therefore, be stated that so far as the total metabolism of
hexoses is concerned, i.e., both respiration and synthesis, glycolysis,
particularly in conjunction with a complementary route such as the
TCA cycle, appears still to be quantitatively a most important avenue in
biological systems.
VI. Direct Oxidations of Glucose and Polyols
A. DIRECT OXIDATION OF GLUCOSE
The direct oxidation of glucose yields gluconic acid. The latter diverts
to a number of pathways, as shown in Fig. 10. Some of these have been
discussed in previous sections. Here emphasis will be made only on
these nonphosphorylative reactions.
The first step in the oxidation of glucose to gluconic acid is through
gluconolactone. Two different classes of enzymes have been found: one
VERNON H. CHELDELIN, CHIH H. WANG, AND TSOO E. KING
tissue ribose (in RNA). He concluded that the oxidative pentose cycle
could not account for the labeling observed. However, Horecker and
Mehler (219) recalculated the results on the assumption that the ribose
had arisen via reversal of the nonoxidative portion of the pentose cycle
and found excellent agreement with the expected values.
A third example, pointing to the importance of the cycle in a synthetic role, comprises the finding (32, 220) that the pentose cycle enzymes are more active and/or abundant during mammalian fetal development, and that in the final one-fourth to one-third of the gestation
period, the fetus develops "adult" characteristics with respect to carbohydrate breakdown, i.e., the glycolysis-Krebs cycle route is greatly preferred in the later stages as well as in postpartum life. These examples
support the idea that the pentose cycle may serve in many organisms
mainly as a pathway for synthesis, even though A. suboxydans (144)
and a few related microorganisms may under appropriate conditions
use this pathway oxidatively, for carbohydrate breakdown. The extent
to which oxidative metabolism of carbohydrates may be routed via the
pentose cycle will depend upon the simultaneous utilizability of other
pathways.
Finally, as suggested in the review by Horecker and Hiatt (2c), a
fourth possible role of the pentose cycle (here operating clockwise, or
in the forward direction) may be to supply TPNH to the system. In
many species, the pentose cycle is TPN-specific; glycolysis and Krebs
cycle oxidations are, on the other hand, largely DPN-requiring (a
possible exception in some systems is isocitric dehydrogenase) so that
processes such as fatty acid synthesis may derive significant amounts of
needed TPNH from phosphogluconate cleavage.
It can, therefore, be stated that so far as the total metabolism of
hexoses is concerned, i.e., both respiration and synthesis, glycolysis,
particularly in conjunction with a complementary route such as the
TCA cycle, appears still to be quantitatively a most important avenue in
biological systems.
VI. Direct Oxidations of Glucose and Polyols
A. DIRECT OXIDATION OF GLUCOSE
The direct oxidation of glucose yields gluconic acid. The latter diverts
to a number of pathways, as shown in Fig. 10. Some of these have been
discussed in previous sections. Here emphasis will be made only on
these nonphosphorylative reactions.
The first step in the oxidation of glucose to gluconic acid is through
gluconolactone. Two different classes of enzymes have been found: one