1. PENTOSE PHOSPHATE CYCLE
35
importance for the synthesis of the ribose moiety of RNA by lymphatic
tissues and tumor.
8. Eyes
Studies of the pentose phosphate cycle in the eye have been concentrated on the cornea, lens, and retina. Source material for these tissues
were the rat, bovine, monkey, human, and rabbit. Most of the work on
the pathways of glucose metabolism in bovine corneal epithelium have
been done by Kinoshita and his co-workers. They showed that both
G-6-P DH and 6-PG DH were present in the homogenate of corneal
epithelium and that pentose phosphate can be converted to sedoheptulose-7-phosphate, from which hexose monophosphate is formed (260).
Using specifically labeled glucose, von Holt et al. concluded that C0 2 production comes almost exclusively via the pentose phosphate cycle (261),
although calculations using the ratio of radioactive lactate from glucosel-C
14 and from glucose-6-C
14 (262) gave a value of 15% for the relative
participation of this pathway for the metabolism of glucose (111). Pyruvate, added in vitro to corneal epithelium, stimulates the preferential
oxidation of C-l of glucose to C0 2 ( 263). The mechanism of this stimulation appears to be due to the presence of a TPN-linked lactate dehydrogenase, thereby coupling the oxidation of glucose with the anaerobic
utilization of pyruvate via the pentose phosphate cycle. Although the
TCA cycle is functioning in the corneal epithelium, the hexose monophosphate shunt appears to be of greater importance in this tissue than
in others, since most of carbon atom number 2 of glucose is oxidized by
the latter pathway (264). Insulin has no effect on the total oxidation and
it does not change the C-l:C-6 ratio of C0 2 (261).
The ocular lens is considered to be metabolically inert because it has
a low rate of oxygen uptake. Nevertheless, if the metabolism of carbohydrate is interfered with, the lens becomes opaque. When isolated rabbit
lenses are kept under conditions that simulate the in situ physiological
state, glucose is converted mainly to lactate (265). As in the case of the
cornea, the pentose phosphate cycle is responsible for the bulk of the
C0 2 formed from the small amount of glucose oxidized by the lens, and
the contribution of this pathway to glucose metabolism is estimated to
be about 2% based on the C-l:C-6 ratio in lactate (111). A high galactose
diet (266) or a high xylose diet (267) fed to rats causes the lenses to
develop cataracts. In the former case, the pentose phosphate cycle
activity diminishes progressively with time, whereas in the latter case,
this activity initially declines, but later returns to the normal level. The
conversion of G-6-P to 6-PG appears to be inhibited by galactose-l-phosphate, and therefore this may be the mechanism for the galactose effect.
35
importance for the synthesis of the ribose moiety of RNA by lymphatic
tissues and tumor.
8. Eyes
Studies of the pentose phosphate cycle in the eye have been concentrated on the cornea, lens, and retina. Source material for these tissues
were the rat, bovine, monkey, human, and rabbit. Most of the work on
the pathways of glucose metabolism in bovine corneal epithelium have
been done by Kinoshita and his co-workers. They showed that both
G-6-P DH and 6-PG DH were present in the homogenate of corneal
epithelium and that pentose phosphate can be converted to sedoheptulose-7-phosphate, from which hexose monophosphate is formed (260).
Using specifically labeled glucose, von Holt et al. concluded that C0 2 production comes almost exclusively via the pentose phosphate cycle (261),
although calculations using the ratio of radioactive lactate from glucosel-C
14 and from glucose-6-C
14 (262) gave a value of 15% for the relative
participation of this pathway for the metabolism of glucose (111). Pyruvate, added in vitro to corneal epithelium, stimulates the preferential
oxidation of C-l of glucose to C0 2 ( 263). The mechanism of this stimulation appears to be due to the presence of a TPN-linked lactate dehydrogenase, thereby coupling the oxidation of glucose with the anaerobic
utilization of pyruvate via the pentose phosphate cycle. Although the
TCA cycle is functioning in the corneal epithelium, the hexose monophosphate shunt appears to be of greater importance in this tissue than
in others, since most of carbon atom number 2 of glucose is oxidized by
the latter pathway (264). Insulin has no effect on the total oxidation and
it does not change the C-l:C-6 ratio of C0 2 (261).
The ocular lens is considered to be metabolically inert because it has
a low rate of oxygen uptake. Nevertheless, if the metabolism of carbohydrate is interfered with, the lens becomes opaque. When isolated rabbit
lenses are kept under conditions that simulate the in situ physiological
state, glucose is converted mainly to lactate (265). As in the case of the
cornea, the pentose phosphate cycle is responsible for the bulk of the
C0 2 formed from the small amount of glucose oxidized by the lens, and
the contribution of this pathway to glucose metabolism is estimated to
be about 2% based on the C-l:C-6 ratio in lactate (111). A high galactose
diet (266) or a high xylose diet (267) fed to rats causes the lenses to
develop cataracts. In the former case, the pentose phosphate cycle
activity diminishes progressively with time, whereas in the latter case,
this activity initially declines, but later returns to the normal level. The
conversion of G-6-P to 6-PG appears to be inhibited by galactose-l-phosphate, and therefore this may be the mechanism for the galactose effect.