1. PENTOSE PHOSPHATE CYCLE
23
this disease (161, 166). In fact, the levels of this enzyme decrease with
increasing severity of arteriosclerosis whereas the level of phosphogluconate dehydrogenase remains more or less constant (161).
6. Other Endocrine Tissues and Related Material
Slices of almost all endocrine glands can oxidize carbon atom 1 of
glucose to C0 2 in preference to carbon atom 6 (147). Among the tissues
examined were ovaries of calf and human. In agreement with these results are the findings by Glock and McLean that enzymes of the oxidative pathway are present in the ovaries of cows (137). However, in
slices of luteinized rat ovaries, very little glucose is metabolized via the
hexose monophosphate shunt (169). This phenomenon applies whether
the glucose uptake by ovaries is stimulated by in vivo injection of luteinizing hormone or not. (Luteinizing hormone, administered to the rat
in vivo, increases the uptake of glucose by the ovaries accompanied by
a less pronounced increase in the production of C
14 0 2 and of C
14
-lipids
from glucose-l-C
14
and glucose-6-C
14
. The rate of incorporation of the
C
14 both into C0 2 and lipids from these two labeled substrates, however,
is the same.) The discrepancy between the two conflicting results was
attributed either to a difference in the proportion of luteal tissues or to
the inactivation of enzymes in the samples studied (169).
In contrast to these studies, experiments with fertilized ovum of a
rabbit using specifically labeled glucose suggested that up to the fourth
day of development (before the blastocyst stage) glucose is mainly oxidized via the pentose phosphate cycle (170). After this stage, the principal pathways are the EMP pathway and TCA cycle. Methylene blue,
however, seemed to stimulate the activity of the pentose phosphate cycle
at this stage.
Histochemical analyses of rabbit pancreas slices showed that glucose6-phosphate dehydrogenase activity is most marked in the beta cells of
islets and in the ductular epithelium (171). The authors concluded from
these studies that glucose-6-phosphate is oxidized predominantly via the
oxidative pathway while the EMP route plays a secondary role. The role
of the pentose phosphate pathway is to furnish TPNH, thus making
available reduced glutathione or free sulfhydryls in enzymes, which in
turn may possibly be connected with the control of insulin output by
the beta cells.
Other workers have shown, also in rabbit pancreas and by histochemical means, that both glucose-6-phosphate dehydrogenase and 6phosphogluconate dehydrogenase exist in the islet and acinar tissues
(172). The glucose-6-phosphate dehydrogenase is present in equal
amounts in islets and acini from pancreas of normal rabbit, but the
23
this disease (161, 166). In fact, the levels of this enzyme decrease with
increasing severity of arteriosclerosis whereas the level of phosphogluconate dehydrogenase remains more or less constant (161).
6. Other Endocrine Tissues and Related Material
Slices of almost all endocrine glands can oxidize carbon atom 1 of
glucose to C0 2 in preference to carbon atom 6 (147). Among the tissues
examined were ovaries of calf and human. In agreement with these results are the findings by Glock and McLean that enzymes of the oxidative pathway are present in the ovaries of cows (137). However, in
slices of luteinized rat ovaries, very little glucose is metabolized via the
hexose monophosphate shunt (169). This phenomenon applies whether
the glucose uptake by ovaries is stimulated by in vivo injection of luteinizing hormone or not. (Luteinizing hormone, administered to the rat
in vivo, increases the uptake of glucose by the ovaries accompanied by
a less pronounced increase in the production of C
14 0 2 and of C
14
-lipids
from glucose-l-C
14
and glucose-6-C
14
. The rate of incorporation of the
C
14 both into C0 2 and lipids from these two labeled substrates, however,
is the same.) The discrepancy between the two conflicting results was
attributed either to a difference in the proportion of luteal tissues or to
the inactivation of enzymes in the samples studied (169).
In contrast to these studies, experiments with fertilized ovum of a
rabbit using specifically labeled glucose suggested that up to the fourth
day of development (before the blastocyst stage) glucose is mainly oxidized via the pentose phosphate cycle (170). After this stage, the principal pathways are the EMP pathway and TCA cycle. Methylene blue,
however, seemed to stimulate the activity of the pentose phosphate cycle
at this stage.
Histochemical analyses of rabbit pancreas slices showed that glucose6-phosphate dehydrogenase activity is most marked in the beta cells of
islets and in the ductular epithelium (171). The authors concluded from
these studies that glucose-6-phosphate is oxidized predominantly via the
oxidative pathway while the EMP route plays a secondary role. The role
of the pentose phosphate pathway is to furnish TPNH, thus making
available reduced glutathione or free sulfhydryls in enzymes, which in
turn may possibly be connected with the control of insulin output by
the beta cells.
Other workers have shown, also in rabbit pancreas and by histochemical means, that both glucose-6-phosphate dehydrogenase and 6phosphogluconate dehydrogenase exist in the islet and acinar tissues
(172). The glucose-6-phosphate dehydrogenase is present in equal
amounts in islets and acini from pancreas of normal rabbit, but the