1. PENTOSE PHOSPHATE CYCLE
37
phosphate cycle as evidenced by the presence of G-6-P DH and 6-PG
DH in rabbit duodenal mucosa (137) and in the intestinal mucosa of
cat, guinea pig, and rat (275). In the mucosa of these last three animals
between 93 and 98% of the activities of the two dehydrogenases is
located in the supernatant fraction, which is free of the nuclei, mitochondria, and microsomes. Furthermore, both dehydrogenases have
about the same specific activities; however, their specific activities were
lower by a factor of 4 than that of lactate dehydrogenase found in the
mucosa. Landau and Wilson have measured the C0 2 C-l: C-6 ratios from
labeled glucose and found that in hamster small intestinal tissue the
ratios averaged 1.5 for the upper jejunum, the midgut, and the lower
ileum (276). The C-l:C-6 lactate ratio from the metabolism of glucose-1C
14
and glucose-6-C
14 in normal rat mucosa of the small intestines is
approximately 0.8 (277). This ratio remained unchanged even after
X-irradiation of the rat (600 roentgens), although the oxidation of the
glucose to C0 2 and lactate increased. The TCA cycle seems to be entirely responsible for this increase while the EMP pathway seems to be
unaffected by the irradiation. The contribution of the pentose phosphate
cycle to the catabolism of glucose by mucosa of both normal and X-irradiated rats was estimated to be 6-8% assuming that no non-triose
phosphate pathway occurs (HI).
10. Kidneys
Aside from the usual renal function of maintaining homeostatic conditions, the kidney appears also to be an actively metabolizing entity.
As it is with other mammalian organs, the kidney has the capacity to
utilize glucose via the oxidative pentose phosphate cycle. This is clearly
indicated by the occurrence of both G-6-P DH and 6-PG DH in the
various tissues of this organ. A summary of the type of tissues, the mammalian sources, and the enzymes studied, as well as the investigators, is
given in Fig. 4. The oxidation of glucose via these enzymes, however, is
probably relatively small since the levels of the dehydrogenases in the
cortex of kidney are less than 10% those found in the liver (279). These
findings correlate well with the results of experiments using variously
labeled glucose. The incorporation of C
14 into C0 2 produced (97, 281)
and into glycogen as well as fatty acids (281) all indicate that the
contribution of the pentose phosphate cycle in the cortex and the
medulla is insignificant.
Alterations in the physiological conditions of the animal may vary
the levels of G-6-P DH and 6-PG DH in the kidney. For example, acute
experimental ischemia caused by unilateral clamping of one renal artery
increased the levels of these two enzymes in the macula densa of the
37
phosphate cycle as evidenced by the presence of G-6-P DH and 6-PG
DH in rabbit duodenal mucosa (137) and in the intestinal mucosa of
cat, guinea pig, and rat (275). In the mucosa of these last three animals
between 93 and 98% of the activities of the two dehydrogenases is
located in the supernatant fraction, which is free of the nuclei, mitochondria, and microsomes. Furthermore, both dehydrogenases have
about the same specific activities; however, their specific activities were
lower by a factor of 4 than that of lactate dehydrogenase found in the
mucosa. Landau and Wilson have measured the C0 2 C-l: C-6 ratios from
labeled glucose and found that in hamster small intestinal tissue the
ratios averaged 1.5 for the upper jejunum, the midgut, and the lower
ileum (276). The C-l:C-6 lactate ratio from the metabolism of glucose-1C
14
and glucose-6-C
14 in normal rat mucosa of the small intestines is
approximately 0.8 (277). This ratio remained unchanged even after
X-irradiation of the rat (600 roentgens), although the oxidation of the
glucose to C0 2 and lactate increased. The TCA cycle seems to be entirely responsible for this increase while the EMP pathway seems to be
unaffected by the irradiation. The contribution of the pentose phosphate
cycle to the catabolism of glucose by mucosa of both normal and X-irradiated rats was estimated to be 6-8% assuming that no non-triose
phosphate pathway occurs (HI).
10. Kidneys
Aside from the usual renal function of maintaining homeostatic conditions, the kidney appears also to be an actively metabolizing entity.
As it is with other mammalian organs, the kidney has the capacity to
utilize glucose via the oxidative pentose phosphate cycle. This is clearly
indicated by the occurrence of both G-6-P DH and 6-PG DH in the
various tissues of this organ. A summary of the type of tissues, the mammalian sources, and the enzymes studied, as well as the investigators, is
given in Fig. 4. The oxidation of glucose via these enzymes, however, is
probably relatively small since the levels of the dehydrogenases in the
cortex of kidney are less than 10% those found in the liver (279). These
findings correlate well with the results of experiments using variously
labeled glucose. The incorporation of C
14 into C0 2 produced (97, 281)
and into glycogen as well as fatty acids (281) all indicate that the
contribution of the pentose phosphate cycle in the cortex and the
medulla is insignificant.
Alterations in the physiological conditions of the animal may vary
the levels of G-6-P DH and 6-PG DH in the kidney. For example, acute
experimental ischemia caused by unilateral clamping of one renal artery
increased the levels of these two enzymes in the macula densa of the