46
Ν. G. PON
glucose-6-C
14 into fatty acids as well as the respired C0 2 , concluded that
the results were not in accords with the sole operation of the Ε MP pathway (123). Their estimate of glucose catabolized via the pentose phosphate cycle was rather high (greater than 50%), but more conservative
values can be obtained from these data (111). Wood and his co-workers
have centered their attention around the synthesis of lactose in the cow's
udder (122, 316). The gist of their efforts is that when cows are injected
with glycerol- 1,3-C
14 into the pudic artery or when the isolated udder
is perfused with either glycerol-l,3-C
14 or glycerol-l,3-C
14 plus glucose2-C
14
, the glucose moiety of the milk lactose arises from the blood glucose, whereas the galactose moiety derived its carbons from glycerol via
a transaldolase exchange reaction. Some evidence for compartmentalization and nonequilibration of hexose phosphate was also presented. In
ketotic cows, data were obtained from measurements of the recovery of
C
14 in respiratory C0 2 , casein, albumin, and citrate that indicate a decreased participation of the pentose phosphate cycle (317). This decrease correlates well with the defect in lipogenesis in the ketotic cow.
As would be expected, the addition of certain types of hormones,
in vitro, affected the metabolic pathways in slices of the mammary glands.
Insulin, for example, increased the yield of radioactive fatty acids derived
from labeled glucose (318) and simultaneously gave a marked stimulation of C
14 0 2 production from glucose-l-C
14
(318, 319). Meanwhile the
production of C
14 0 2 from glucose-6-C
14
is diminished and that from
glucose-3,4-C
14 remains unaltered (318). Since this hormone does not
preferentially enhance the incorporation of radioactivity into fatty acids
from glucose labeled at C-l and C-6, the conclusion is that the alternate
oxidative pathway is not stimulated.
Hormones of the anterior pituitary glands also have some similar
effects to those with insulin. For example, growth hormone and ACTH
elevate glucose oxidation of both C-l and C-6 throughout lactation
(320). Similarly, prolactin affects the oxidation of glucose-l-C
14
, but it
has a minimal effect on the oxidation of glucose-6-C
14 in the late stage
of lactation. In contrast to these results, McLean (321) found that prolactin had no consistent effect on the metabolism of specifically labeled
glucose by mammary gland slices from lactating rats; although she
observed an increase in the formation of C
14 0 2 from both glucose-l-C
14
and from glucose-6-C
14
. Furthermore, there was an increased incorporation of both glucose carbons into the lipids by slices from pregnant rats,
due to in vitro addition of prolactin. The discrepancy in the prolactin
effect on the oxidation of variously labeled glucose between these two
groups of investigators has yet to be resolved.
Other hormones of the anterior pituitary gland have also been tested
Ν. G. PON
glucose-6-C
14 into fatty acids as well as the respired C0 2 , concluded that
the results were not in accords with the sole operation of the Ε MP pathway (123). Their estimate of glucose catabolized via the pentose phosphate cycle was rather high (greater than 50%), but more conservative
values can be obtained from these data (111). Wood and his co-workers
have centered their attention around the synthesis of lactose in the cow's
udder (122, 316). The gist of their efforts is that when cows are injected
with glycerol- 1,3-C
14 into the pudic artery or when the isolated udder
is perfused with either glycerol-l,3-C
14 or glycerol-l,3-C
14 plus glucose2-C
14
, the glucose moiety of the milk lactose arises from the blood glucose, whereas the galactose moiety derived its carbons from glycerol via
a transaldolase exchange reaction. Some evidence for compartmentalization and nonequilibration of hexose phosphate was also presented. In
ketotic cows, data were obtained from measurements of the recovery of
C
14 in respiratory C0 2 , casein, albumin, and citrate that indicate a decreased participation of the pentose phosphate cycle (317). This decrease correlates well with the defect in lipogenesis in the ketotic cow.
As would be expected, the addition of certain types of hormones,
in vitro, affected the metabolic pathways in slices of the mammary glands.
Insulin, for example, increased the yield of radioactive fatty acids derived
from labeled glucose (318) and simultaneously gave a marked stimulation of C
14 0 2 production from glucose-l-C
14
(318, 319). Meanwhile the
production of C
14 0 2 from glucose-6-C
14
is diminished and that from
glucose-3,4-C
14 remains unaltered (318). Since this hormone does not
preferentially enhance the incorporation of radioactivity into fatty acids
from glucose labeled at C-l and C-6, the conclusion is that the alternate
oxidative pathway is not stimulated.
Hormones of the anterior pituitary glands also have some similar
effects to those with insulin. For example, growth hormone and ACTH
elevate glucose oxidation of both C-l and C-6 throughout lactation
(320). Similarly, prolactin affects the oxidation of glucose-l-C
14
, but it
has a minimal effect on the oxidation of glucose-6-C
14 in the late stage
of lactation. In contrast to these results, McLean (321) found that prolactin had no consistent effect on the metabolism of specifically labeled
glucose by mammary gland slices from lactating rats; although she
observed an increase in the formation of C
14 0 2 from both glucose-l-C
14
and from glucose-6-C
14
. Furthermore, there was an increased incorporation of both glucose carbons into the lipids by slices from pregnant rats,
due to in vitro addition of prolactin. The discrepancy in the prolactin
effect on the oxidation of variously labeled glucose between these two
groups of investigators has yet to be resolved.
Other hormones of the anterior pituitary gland have also been tested