1. PENTOSE PHOSPHATE CYCLE
27
the role of the pentose phosphate cycle in the reduction of methemoglobin, also using uniformly labeled glucose
(192).
Experiments using enzymatic assays for measuring the role of the
pentose cycle in erythrocytic metabolism of glucose are far too numerous
to mention. Of all the enzymes, glucose-6-phosphate dehydrogenase
appears to be the most frequently studied although attention has been
focused on other enzymes of the pentose cycle. Some relatively recent
results have been derived from experiments on the effect of steroids on
glucose-6-phosphate and 6-phosphogluconate dehydrogenase activity
(193), monkey erythrocytic glucose-6-phosphate dehydrogenase
(194),
and the nonoxidative enzymes of the pentose phosphate cycle in the
human erythrocyte
(195-197).
Products formed in various substrates have been studied extensively
also. Late examples of these investigations have been documented by
Bucolo and Bartlett (198), Dische and Igals (199), Lionetti et al. (200),
Golovatsky (201), and Shafer and Bartlett (202). In general, either
ribose in the form of nucleosides or as the phosphate was added to
erythrocytes, their ghosts, or hemolyzates. Products were examined
colorimetrically, enzymatically, and chromatographically.
Most of the research on glucose metabolism has been performed on
red blood cells from humans. Other animals, however, have served as
sources; e.g., cows (201), sheep (201), monkeys (194), cats
(194),
horses (21), rabbits (201, 203), and rats (204). The extent of oxidation
of glucose via the pentose phosphate pathway is of the order of 10% as
calculated by Murphy (190) and by Katz (111), who used the data of
Brin and Yonemoto (189). This contribution by the shunt mechanism,
however, is dependent on the presence or absence of methylene blue
(189), on the pH of the suspending medium (190), on the oxygen tension (190), and on the age of the erythrocytes (205). In connection with
this last item, aged erythrocytes have lower activities of glucoses-phosphate and 6-phosphogluconate dehydrogenases than young cells. Also
related to this problem are the findings by Rubenstein et al., that the
reticulocyte, the presumed nucleated precursor of the erythrocyte in
hemopoiesis, has about the same level of glucose-6-phosphate dehydrogenase as the mature erythrocyte (203).
Besides the two dehydrogenases, the nonoxidative enzymes have been
found in erythrocytes and characterized. Phosphoribose isomerase has
been shown to exist in human erythrocytes (195) and in a whole gamut
of erythrocytes of different species (206), which include the enzyme, in
decreasing order of activity, from cats, dogs, horses, pigs, bovines, mice
(whole blood in this case), rats, sheep, guinea pigs, and fowl. Human
27
the role of the pentose phosphate cycle in the reduction of methemoglobin, also using uniformly labeled glucose
(192).
Experiments using enzymatic assays for measuring the role of the
pentose cycle in erythrocytic metabolism of glucose are far too numerous
to mention. Of all the enzymes, glucose-6-phosphate dehydrogenase
appears to be the most frequently studied although attention has been
focused on other enzymes of the pentose cycle. Some relatively recent
results have been derived from experiments on the effect of steroids on
glucose-6-phosphate and 6-phosphogluconate dehydrogenase activity
(193), monkey erythrocytic glucose-6-phosphate dehydrogenase
(194),
and the nonoxidative enzymes of the pentose phosphate cycle in the
human erythrocyte
(195-197).
Products formed in various substrates have been studied extensively
also. Late examples of these investigations have been documented by
Bucolo and Bartlett (198), Dische and Igals (199), Lionetti et al. (200),
Golovatsky (201), and Shafer and Bartlett (202). In general, either
ribose in the form of nucleosides or as the phosphate was added to
erythrocytes, their ghosts, or hemolyzates. Products were examined
colorimetrically, enzymatically, and chromatographically.
Most of the research on glucose metabolism has been performed on
red blood cells from humans. Other animals, however, have served as
sources; e.g., cows (201), sheep (201), monkeys (194), cats
(194),
horses (21), rabbits (201, 203), and rats (204). The extent of oxidation
of glucose via the pentose phosphate pathway is of the order of 10% as
calculated by Murphy (190) and by Katz (111), who used the data of
Brin and Yonemoto (189). This contribution by the shunt mechanism,
however, is dependent on the presence or absence of methylene blue
(189), on the pH of the suspending medium (190), on the oxygen tension (190), and on the age of the erythrocytes (205). In connection with
this last item, aged erythrocytes have lower activities of glucoses-phosphate and 6-phosphogluconate dehydrogenases than young cells. Also
related to this problem are the findings by Rubenstein et al., that the
reticulocyte, the presumed nucleated precursor of the erythrocyte in
hemopoiesis, has about the same level of glucose-6-phosphate dehydrogenase as the mature erythrocyte (203).
Besides the two dehydrogenases, the nonoxidative enzymes have been
found in erythrocytes and characterized. Phosphoribose isomerase has
been shown to exist in human erythrocytes (195) and in a whole gamut
of erythrocytes of different species (206), which include the enzyme, in
decreasing order of activity, from cats, dogs, horses, pigs, bovines, mice
(whole blood in this case), rats, sheep, guinea pigs, and fowl. Human