48
Ν. G. PON
the oxidation of the glucose C-l. The data suggest that the rate-limiting
factor in the oxidation of glucose via the pentose phosphate cycle is the
reoxidation of TPNH. This notion is supported by a previous finding
that phenazine methosulfate, an artificial electron acceptor, can increase
the production of C
14 0 2 from glucose-l-C
14
(319). A point should be
emphasized here—that the conversion of glucose-l-H
3
to fatty acids
increases as lactation progresses whereas the conversion of lactate-2-H
3
to fatty acids decreases (300). These results indicate that the bulk of the
TPNH arising from the pentose phosphate cycle is responsible for the
synthesis of fatty acids, in contrast to the results from work on the liver.
The fact that the weight of the mammary gland is constant throughout lactation, after correcting for the milk content, and that the dehydrogenase activities increase during this time, suggests that the high
dehydrogenase activities are associated with milk secretion rather than
growth (310). In accord with this conclusion are the findings of experiments with hormonally induced lactating mammary glands from hypophysectomized rats (322), but with a further conjecture that the metabolic pattern is connected also with milk formation. Finally, Goodfriend
and Topper suggested that the physiological effect of oxytocin on mammary glands may be related to the direct stimulation of metabolism in
the secreting cells (315).
13. Muscle
Of all the mammalian tissues studied, there seems to be little doubt
that muscle tissue metabolizes glucose almost exclusively by way of the
EMP route. This fact has been established in rat diaphragm by isotopic
techniques (97, 111, 324). The potential of this tissue to utilize glucose
via the pentose phosphate cycle still exists, nevertheless, since the pentose
phosphate cycle enzymes are present in this tissue (111). Although
adrenalectomy enhances the incorporation of C
14 from glucose-l-C
14 and
from glucose-6-C
14 into proteins of isolated diaphragm, the proportion
of the C-l and C-6 incorporated into protein is unchanged (324). Under
anaerobic conditions, however, pyruvate stimulates the production of
C0 2 from the C-l of glucose without affecting that from C-6 of glucose
(325). The mechanism of this activation may be through a TPN-linked
lactate dehydrogenase system. It is conceivable that under aerobic conditions, the activity of the EMP pathway far overshadows that of the
pentose phosphate cycle. Skeletal muscle also has G-6-P DH and 6-PG
DH in rats and mice (137), along with phosphopentose isomerase,
D-ribulose-5-phosphate-3-epimerase in rabbits (326), transketolase in
rabbits (327), and the necessary enzymes in rats to convert fructose-6phosphate to heptose phosphate in the presence of fructose-l,6-diphos-
Ν. G. PON
the oxidation of the glucose C-l. The data suggest that the rate-limiting
factor in the oxidation of glucose via the pentose phosphate cycle is the
reoxidation of TPNH. This notion is supported by a previous finding
that phenazine methosulfate, an artificial electron acceptor, can increase
the production of C
14 0 2 from glucose-l-C
14
(319). A point should be
emphasized here—that the conversion of glucose-l-H
3
to fatty acids
increases as lactation progresses whereas the conversion of lactate-2-H
3
to fatty acids decreases (300). These results indicate that the bulk of the
TPNH arising from the pentose phosphate cycle is responsible for the
synthesis of fatty acids, in contrast to the results from work on the liver.
The fact that the weight of the mammary gland is constant throughout lactation, after correcting for the milk content, and that the dehydrogenase activities increase during this time, suggests that the high
dehydrogenase activities are associated with milk secretion rather than
growth (310). In accord with this conclusion are the findings of experiments with hormonally induced lactating mammary glands from hypophysectomized rats (322), but with a further conjecture that the metabolic pattern is connected also with milk formation. Finally, Goodfriend
and Topper suggested that the physiological effect of oxytocin on mammary glands may be related to the direct stimulation of metabolism in
the secreting cells (315).
13. Muscle
Of all the mammalian tissues studied, there seems to be little doubt
that muscle tissue metabolizes glucose almost exclusively by way of the
EMP route. This fact has been established in rat diaphragm by isotopic
techniques (97, 111, 324). The potential of this tissue to utilize glucose
via the pentose phosphate cycle still exists, nevertheless, since the pentose
phosphate cycle enzymes are present in this tissue (111). Although
adrenalectomy enhances the incorporation of C
14 from glucose-l-C
14 and
from glucose-6-C
14 into proteins of isolated diaphragm, the proportion
of the C-l and C-6 incorporated into protein is unchanged (324). Under
anaerobic conditions, however, pyruvate stimulates the production of
C0 2 from the C-l of glucose without affecting that from C-6 of glucose
(325). The mechanism of this activation may be through a TPN-linked
lactate dehydrogenase system. It is conceivable that under aerobic conditions, the activity of the EMP pathway far overshadows that of the
pentose phosphate cycle. Skeletal muscle also has G-6-P DH and 6-PG
DH in rats and mice (137), along with phosphopentose isomerase,
D-ribulose-5-phosphate-3-epimerase in rabbits (326), transketolase in
rabbits (327), and the necessary enzymes in rats to convert fructose-6phosphate to heptose phosphate in the presence of fructose-l,6-diphos-