1. PENTOSE PHOSPHATE CYCLE
29
added to the blood for these studies may have been inadequate for the
thyrotoxic blood.
Glucose-6-phosphate dehydrogenase, being a key enzyme of the
oxidative portion of the pentose phosphate cycle, has borne the brunt
of attack from innumerable researchers. A comprehensive review of this
enzyme, as well as of 6-phosphogluconate dehydrogenase, has now been
made available (213). Since glucose-6-phosphate dehydrogenase is a
TPN
+
-linked enzyme, it is not surprising that this coenzyme has the
greatest effect on the activity of the enzyme. However, TPNH also stimulates the metabolism of glucose-l-C
14
to C
14 0 2 , whereas DPN
+
and
DPNH are only mildly stimulatory (214). The activation of TPNH may
be due to the action of a transhydrogenase. The resulting DPNH is
reconverted to DPN
+ by the lactate dehydrogenase reaction. Glucose-6phosphate dehydrogenase from normal human erythrocytes apparently
can exist in a partially active state or an almost totally inactive state
(215, 216). In the presence of TPN
+ , the enzyme is reactivated and there
are accompanying changes in sedimentation properties and antigenic
activity.
As noted before, the effects of steroids on dehydrogenase activity
have been studied (193). Very low concentrations (<10~
6 M) inhibit
glucose-6-phosphate dehydrogenase from mammalian tissues including
human erythrocytes; that from yeast or spinach is not affected. Even
mammalian 6-phosphogluconate dehydrogenase remains uninhibited.
Furthermore, only steroids possessing a ketone group in the C-17 or
C-20 positions are effective; e.g., dehydroisoandrosterone or pregnenolone. Steroids such as estrogens, testosterone, corticosteroids, and progesterone were ineffective as inhibitors in concentrations up to 4 χ ΙΟ
-5 M.
Other chemicals that apparently affect the oxidative enzymes of the
pentose phosphate cycle are cysteine, ascorbate, pyruvate, and a number
of hemolytic anemia-inducing drugs such as acetylphenylhydrazine,
nitrofurantoin, a- and ß-naphthol, primaquine phosphate, and fava bean
extract (217). Addition of any one of these substances to red blood cells
of normal human subjects enhances the oxidation rate of glucose-l-C
14
to C
14 0 2 , but does not increase the rate of conversion of glucose to
lactate. The stimulatory effect is due not to the direct action on glucose6-phosphate dehydrogenase, but to the oxidation of TPNH in the presence of diaphorase and the substance added. In fact, α-naphthol and
nitrofurantoin both inhibit glucose-6-phosphate dehydrogenase. The increased rate of reoxidation of TPNH may be a factor in the mechanism
by which these substances destroy aged red blood cells.
Certain pathological conditions also affect the levels of the oxidative
enzymes of the shunt. Thus reticulocytosis as a consequence of acquired
29
added to the blood for these studies may have been inadequate for the
thyrotoxic blood.
Glucose-6-phosphate dehydrogenase, being a key enzyme of the
oxidative portion of the pentose phosphate cycle, has borne the brunt
of attack from innumerable researchers. A comprehensive review of this
enzyme, as well as of 6-phosphogluconate dehydrogenase, has now been
made available (213). Since glucose-6-phosphate dehydrogenase is a
TPN
+
-linked enzyme, it is not surprising that this coenzyme has the
greatest effect on the activity of the enzyme. However, TPNH also stimulates the metabolism of glucose-l-C
14
to C
14 0 2 , whereas DPN
+
and
DPNH are only mildly stimulatory (214). The activation of TPNH may
be due to the action of a transhydrogenase. The resulting DPNH is
reconverted to DPN
+ by the lactate dehydrogenase reaction. Glucose-6phosphate dehydrogenase from normal human erythrocytes apparently
can exist in a partially active state or an almost totally inactive state
(215, 216). In the presence of TPN
+ , the enzyme is reactivated and there
are accompanying changes in sedimentation properties and antigenic
activity.
As noted before, the effects of steroids on dehydrogenase activity
have been studied (193). Very low concentrations (<10~
6 M) inhibit
glucose-6-phosphate dehydrogenase from mammalian tissues including
human erythrocytes; that from yeast or spinach is not affected. Even
mammalian 6-phosphogluconate dehydrogenase remains uninhibited.
Furthermore, only steroids possessing a ketone group in the C-17 or
C-20 positions are effective; e.g., dehydroisoandrosterone or pregnenolone. Steroids such as estrogens, testosterone, corticosteroids, and progesterone were ineffective as inhibitors in concentrations up to 4 χ ΙΟ
-5 M.
Other chemicals that apparently affect the oxidative enzymes of the
pentose phosphate cycle are cysteine, ascorbate, pyruvate, and a number
of hemolytic anemia-inducing drugs such as acetylphenylhydrazine,
nitrofurantoin, a- and ß-naphthol, primaquine phosphate, and fava bean
extract (217). Addition of any one of these substances to red blood cells
of normal human subjects enhances the oxidation rate of glucose-l-C
14
to C
14 0 2 , but does not increase the rate of conversion of glucose to
lactate. The stimulatory effect is due not to the direct action on glucose6-phosphate dehydrogenase, but to the oxidation of TPNH in the presence of diaphorase and the substance added. In fact, α-naphthol and
nitrofurantoin both inhibit glucose-6-phosphate dehydrogenase. The increased rate of reoxidation of TPNH may be a factor in the mechanism
by which these substances destroy aged red blood cells.
Certain pathological conditions also affect the levels of the oxidative
enzymes of the shunt. Thus reticulocytosis as a consequence of acquired