28
Ν. G. PON
erythrocyte phosphoriboisomerase was the most active. Bruns et al.
(206) also showed that phosphoriboisomerase of mouse erythrocytes has
the highest activity as compared with that of heart muscle, skeletal
muscle, liver, or kidney. Pig hemolyzates possess a complete pentose phosphate cycle, the ketopentoses being formed at a greater rate than triose
phosphate and sedoheptulose-7-phosphate followed by fructose-6-phosphate and glucose-6-phosphate. These results are in contrast to those
obtained by Schneider and Wagner, who were unable to demonstrate the
presence of glucose-6-phosphate dehydrogenase in pig erythrocytes
(188).
Transketolase in pentose metabolism has been implicated by experiments with thiamine-deficient rat erythrocytes (204). In these erythrocytes, in the presence of methylene blue, pentose accumulates to levels
greater than normal and the oxidation of glucose-2-C
14
to C
14 0 2 is
strikingly depressed below normal. This latter fact indicates that recycling of glucose is hindered by the thiamine deficiency. Thiamine
therapy, on the other hand, decreases the pentose level in human red
blood cells (207). Because the action of transketolase is mediated by
thiamine pyrophosphate (208) one should expect to find this enzyme in
erythrocytes, and indeed it is present (196, 209).
When intact erythrocytes are incubated with inosine, the product is
sedoheptulose-l,7-diphosphate. This product is probably formed by an
aldolase reaction of dihydroxyacetone phosphate and erythrose-4-phosphate (198). Furthermore, in the conversion of ribose-5-phosphate to
hexose-6-phosphate by human hemolyzates, sedoheptulose and triose
phosphates are intermediates (210).
The fact that the tetrose phosphate has to be present for the synthesis
of sedoheptulose-l,7-diphosphate indicates that transaldolase occurs in
the red blood cell. This has been demonstrated by experiments with
stroma-free hemolyzates of human erythrocytes (196).
Several factors that influence the pentose phosphate cycle in red
blood cells have already been mentioned, such as the presence of
methylene blue, the age of the erythrocytes, and the effect of thiamine.
Blood of individuals with hyperthyroid conditions and subjects with
experimentally induced (by treatment with triiodothyronine) hyperthyroidism, gave a decrease in the early C
14 0 2 production from glucose-1C
14 in the presence of methylene blue (191). The reason for this phenomenon is obscure because the levels of both glucose-6-phosphate and
6-phosphogluconate dehydrogenases are actually raised in thyrotoxic
cases (211, 212). Some explanations were forwarded: There may be a
net decrease in both TPN
+ and TPNH levels; other enzymes of the pentose phosphate cycle may have diminished; and/or the methylene blue
Ν. G. PON
erythrocyte phosphoriboisomerase was the most active. Bruns et al.
(206) also showed that phosphoriboisomerase of mouse erythrocytes has
the highest activity as compared with that of heart muscle, skeletal
muscle, liver, or kidney. Pig hemolyzates possess a complete pentose phosphate cycle, the ketopentoses being formed at a greater rate than triose
phosphate and sedoheptulose-7-phosphate followed by fructose-6-phosphate and glucose-6-phosphate. These results are in contrast to those
obtained by Schneider and Wagner, who were unable to demonstrate the
presence of glucose-6-phosphate dehydrogenase in pig erythrocytes
(188).
Transketolase in pentose metabolism has been implicated by experiments with thiamine-deficient rat erythrocytes (204). In these erythrocytes, in the presence of methylene blue, pentose accumulates to levels
greater than normal and the oxidation of glucose-2-C
14
to C
14 0 2 is
strikingly depressed below normal. This latter fact indicates that recycling of glucose is hindered by the thiamine deficiency. Thiamine
therapy, on the other hand, decreases the pentose level in human red
blood cells (207). Because the action of transketolase is mediated by
thiamine pyrophosphate (208) one should expect to find this enzyme in
erythrocytes, and indeed it is present (196, 209).
When intact erythrocytes are incubated with inosine, the product is
sedoheptulose-l,7-diphosphate. This product is probably formed by an
aldolase reaction of dihydroxyacetone phosphate and erythrose-4-phosphate (198). Furthermore, in the conversion of ribose-5-phosphate to
hexose-6-phosphate by human hemolyzates, sedoheptulose and triose
phosphates are intermediates (210).
The fact that the tetrose phosphate has to be present for the synthesis
of sedoheptulose-l,7-diphosphate indicates that transaldolase occurs in
the red blood cell. This has been demonstrated by experiments with
stroma-free hemolyzates of human erythrocytes (196).
Several factors that influence the pentose phosphate cycle in red
blood cells have already been mentioned, such as the presence of
methylene blue, the age of the erythrocytes, and the effect of thiamine.
Blood of individuals with hyperthyroid conditions and subjects with
experimentally induced (by treatment with triiodothyronine) hyperthyroidism, gave a decrease in the early C
14 0 2 production from glucose-1C
14 in the presence of methylene blue (191). The reason for this phenomenon is obscure because the levels of both glucose-6-phosphate and
6-phosphogluconate dehydrogenases are actually raised in thyrotoxic
cases (211, 212). Some explanations were forwarded: There may be a
net decrease in both TPN
+ and TPNH levels; other enzymes of the pentose phosphate cycle may have diminished; and/or the methylene blue