1. PENTOSE PHOSPHATE CYCLE
21
of TPNH; however, if the level is limiting, then the steroid stimulates
the production of C0 2 (142). Most likely, steroid is in the meantime
being reduced to the corresponding Cortisol, as in the case of progesterone added in vitro to a similar system (143). In vitro addition of
thioperazine inhibits both dehydrogenases of the pentose cycle—glucose
phosphate enzyme noncompetitively, and phosphogluconate enzyme competitively (144). On the other hand, ACTH administered either in vivo
(145,146) or in vitro (147) had no effect on the dehydrogenase systems,
but instead decreased the specific activity and total activity of glucose-6phosphatase (145, 146). Finally, the levels of glucose-6-phosphate dehydrogenase, transketolase, and transaldolase were not modified by pregnancy (148). Thus far, adrenals from human fetuses, rats, oxen, calfs,
guinea pigs, and mice have been examined.
4. Brain and the Nervous System
Experiments with variously labeled glucose led to the conclusion that
in cerebral cortex slices, the EMP pathway is almost totally operative in
the catabolism of glucose (149-151).
Nevertheless, enzymes of the pentose phosphate cycle, such as transketolase, and the dehydrogenases do
exist in homogenates of the brains from various sources (137, 152-155).
The oxidation of pentose phosphate cycle intermediates and of labeled
glucose is stimulated by addition of TPN
+ in vitro (151, 156, 157). Higgins (149) surmised that the dehydrogenases were carried over from
the young stage where cholesterol and lipid synthesis occur in situ,
thereby oxidizing TPNH without the presence of oxygen. On the other
hand, Hotta (157) feels that a considerable amount of TPN
+ is lost
during the preparation of the tissues for the experiments. That oxidized
glutathione also accelerates the oxidation of glucose-l-C
14 to C
14 0 2 but
not that of glucose-6-C
14 suggests that the pentose phosphate pathway is
responsible for the reduction of oxidized glutathione, the reduced form
being necessary for the protection of the integrity of the cells.
A study of the distribution of glucose-6-phosphate dehydrogenase and
of transketolase in the brain and the spinal cord showed that these enzymes are found in both regions (152,154,155).
Levels of these enzymes
are higher in the spinal cord than in the brain. Moreover, the white
matter of the central nervous system tends to be more active than the
gray matter, indicating that perhaps the pentose phosphate cycle may be
connected with the initial development and subsequent maintenance of
the normal myelin sheath. Indeed, the heavily myelinated tract with the
highest lipid content as well as the highest glucose-6-phosphate dehydrogenase activity is the dorsal column (155).
The intracellular localization of the dehydrogenases has been dis-
21
of TPNH; however, if the level is limiting, then the steroid stimulates
the production of C0 2 (142). Most likely, steroid is in the meantime
being reduced to the corresponding Cortisol, as in the case of progesterone added in vitro to a similar system (143). In vitro addition of
thioperazine inhibits both dehydrogenases of the pentose cycle—glucose
phosphate enzyme noncompetitively, and phosphogluconate enzyme competitively (144). On the other hand, ACTH administered either in vivo
(145,146) or in vitro (147) had no effect on the dehydrogenase systems,
but instead decreased the specific activity and total activity of glucose-6phosphatase (145, 146). Finally, the levels of glucose-6-phosphate dehydrogenase, transketolase, and transaldolase were not modified by pregnancy (148). Thus far, adrenals from human fetuses, rats, oxen, calfs,
guinea pigs, and mice have been examined.
4. Brain and the Nervous System
Experiments with variously labeled glucose led to the conclusion that
in cerebral cortex slices, the EMP pathway is almost totally operative in
the catabolism of glucose (149-151).
Nevertheless, enzymes of the pentose phosphate cycle, such as transketolase, and the dehydrogenases do
exist in homogenates of the brains from various sources (137, 152-155).
The oxidation of pentose phosphate cycle intermediates and of labeled
glucose is stimulated by addition of TPN
+ in vitro (151, 156, 157). Higgins (149) surmised that the dehydrogenases were carried over from
the young stage where cholesterol and lipid synthesis occur in situ,
thereby oxidizing TPNH without the presence of oxygen. On the other
hand, Hotta (157) feels that a considerable amount of TPN
+ is lost
during the preparation of the tissues for the experiments. That oxidized
glutathione also accelerates the oxidation of glucose-l-C
14 to C
14 0 2 but
not that of glucose-6-C
14 suggests that the pentose phosphate pathway is
responsible for the reduction of oxidized glutathione, the reduced form
being necessary for the protection of the integrity of the cells.
A study of the distribution of glucose-6-phosphate dehydrogenase and
of transketolase in the brain and the spinal cord showed that these enzymes are found in both regions (152,154,155).
Levels of these enzymes
are higher in the spinal cord than in the brain. Moreover, the white
matter of the central nervous system tends to be more active than the
gray matter, indicating that perhaps the pentose phosphate cycle may be
connected with the initial development and subsequent maintenance of
the normal myelin sheath. Indeed, the heavily myelinated tract with the
highest lipid content as well as the highest glucose-6-phosphate dehydrogenase activity is the dorsal column (155).
The intracellular localization of the dehydrogenases has been dis-