36
Ν. G. PON
The xylose effect is less clear, but the formation and utilization of Dxylulose is implicated. Certain pathological conditions also affect the
activity of the pentose phosphate pathway. For example, in human
subjects with primaquine-sensitive erythrocytes, there is a marked reduction of the lens G-6-P DH although the level of 6-PG DH is normal
(268). Cataractous lens from these individuals also has a lower G-6-P
DH content than normal while the 6-PG DH level is normal. On the
other hand, cataractous lenses from humans with normal G-6-P DH in
their erythrocytes have higher specific activities of both dehydrogenases,
the more advanced the cataract, the higher the specific activities. AlIoxan-induced diabetes decreases the formation of C
14 0 2 from glucose-1C
14 by rat lens whereas that from glucose-6-C
14 remains unaffected (269).
Whereas insulin added in vitro to normal rat lens suspended in KrebsRinger bicarbonate buffer stimulates C-l oxidation and inhibits C-6
oxidation to C0 2 , insulin diminishes the former oxidation without affecting
the latter in lens from the alloxan diabetic rat. [Insulin has no effect on
either C-l or C-6 oxidation when this tissue is suspended in Krebs-Ringer
phosphate buffer. In contrast to these findings, von Holt et al. found that
insulin had no effect on the total oxidation of glucose and did not alter
the C-l:C-6 ratio of C0 2 (261).]
The utilization of glucose in cattle retina is almost exclusively via the
EMP and TCA pathways (270). Under anaerobic conditions, however,
the hexose monophosphate pathway is activated in intact retina in the
presence of pyruvate. Rahman and Kerly, using the C-l:C-6 ratios of
specific activity of lactate, concluded that in ox retina 1 of 4 moles of
glucose is metabolized via the pentose phosphate cycle (271). G-6-P DH
and 6-PH DH have been found in bovine, rabbit, and monkey (Macaca
rhesus) retina (272, 273). Both dehydrogenases have the same specific
activities in the cow's retina (272). Histochemical analyses showed that
G-6-P DH is very rich throughout most of the first neuron of the retina
of rabbit as well as monkey (273), the activity of this enzyme being
about 10-30 times greater than that of the rest of the retina. The localization of DH activity of 6-PG parallels that of G-6-P DH, but at lower
levels. Insulin added to retina significantly increases glucose oxidation
but did not change the C-l:C-6 C0 2 (261).
9. Intestines
This highly specialized and functional organ consists of connective
tissues, smooth muscles, and the mucosa, this last category making up
one-fifth of the total mass of the intestines (274). In addition, the mucosa
is an actively regenerating tissue, one-half of the cells turning over every
2 or 3 days. In this tissue there is a potentially active oxidative pentose
Ν. G. PON
The xylose effect is less clear, but the formation and utilization of Dxylulose is implicated. Certain pathological conditions also affect the
activity of the pentose phosphate pathway. For example, in human
subjects with primaquine-sensitive erythrocytes, there is a marked reduction of the lens G-6-P DH although the level of 6-PG DH is normal
(268). Cataractous lens from these individuals also has a lower G-6-P
DH content than normal while the 6-PG DH level is normal. On the
other hand, cataractous lenses from humans with normal G-6-P DH in
their erythrocytes have higher specific activities of both dehydrogenases,
the more advanced the cataract, the higher the specific activities. AlIoxan-induced diabetes decreases the formation of C
14 0 2 from glucose-1C
14 by rat lens whereas that from glucose-6-C
14 remains unaffected (269).
Whereas insulin added in vitro to normal rat lens suspended in KrebsRinger bicarbonate buffer stimulates C-l oxidation and inhibits C-6
oxidation to C0 2 , insulin diminishes the former oxidation without affecting
the latter in lens from the alloxan diabetic rat. [Insulin has no effect on
either C-l or C-6 oxidation when this tissue is suspended in Krebs-Ringer
phosphate buffer. In contrast to these findings, von Holt et al. found that
insulin had no effect on the total oxidation of glucose and did not alter
the C-l:C-6 ratio of C0 2 (261).]
The utilization of glucose in cattle retina is almost exclusively via the
EMP and TCA pathways (270). Under anaerobic conditions, however,
the hexose monophosphate pathway is activated in intact retina in the
presence of pyruvate. Rahman and Kerly, using the C-l:C-6 ratios of
specific activity of lactate, concluded that in ox retina 1 of 4 moles of
glucose is metabolized via the pentose phosphate cycle (271). G-6-P DH
and 6-PH DH have been found in bovine, rabbit, and monkey (Macaca
rhesus) retina (272, 273). Both dehydrogenases have the same specific
activities in the cow's retina (272). Histochemical analyses showed that
G-6-P DH is very rich throughout most of the first neuron of the retina
of rabbit as well as monkey (273), the activity of this enzyme being
about 10-30 times greater than that of the rest of the retina. The localization of DH activity of 6-PG parallels that of G-6-P DH, but at lower
levels. Insulin added to retina significantly increases glucose oxidation
but did not change the C-l:C-6 C0 2 (261).
9. Intestines
This highly specialized and functional organ consists of connective
tissues, smooth muscles, and the mucosa, this last category making up
one-fifth of the total mass of the intestines (274). In addition, the mucosa
is an actively regenerating tissue, one-half of the cells turning over every
2 or 3 days. In this tissue there is a potentially active oxidative pentose