9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
435
enzyme (specific activity = 86) required several hundredfold concentration. The bacterial enzyme employs either pyridine nucleotide as
cofactor, whereas that from mammalian or plant sources tends to be
specific for TPN
2 . The isoelectric point for the enzyme from frog muscle (17) is reported to be at pH 4.8, whereas that from rat blood is
5.85 (17a). Bivalent ions have been shown to activate the enzyme from
yeast and Escherichia colt, but this has not been observed with G-6-P
dehydrogenase from other sources.
3. 6-Phosphogluconate
Dehydrogenase
This enzyme, like glucose-6-phosphate dehydrogenase, has been reported active in many of the systems referred to in the preceding section; this is to be expected since 6-phosphogluconate is the product of
G-6-P oxidation.
6-PGA dehydrogenase is further similar to G-6-P dehydrogenase in
that its purification has been emphasized from microbial systems. Extensive purification of 6-PGA dehydrogenase has been carried out using
yeast (86), Leuconostoc (57), E. colt (52), and Acetobacter suboxydans (66). Plant materials, although mentioned as a source of this
enzyme (25, 51, 87), have not been extensively exploited. The enzyme
from A. sub oxydans has been purified most by means of electrophoresis
on a starch column, so that it is free from G-6-P dehydrogenase (66,
88). Table II contains some comparisons of the enzyme from different
sources.
The oxidation of 6-PGA by TPN involves both a decarboxylation
and a dehydrogenation. As DeLey and Verhofstede (89, 90) have
pointed out, the explanation for the mechanism of this reaction has
shifted back and forth between the speculation that either 2-keto- or
3-keto-6-PGA was formed or that 2, 3-enediol gluconolactone-6-phosphate might also be an intermediate. The idea that 2-keto-6PGA served
as the intermediate was attractive also to Uehara and Akabori (91, 92),
who suggested that 2-keto-6-PGA might undergo an aldolase cleavage
to hydroxypyruvate and glyceraldehyde-3-phosphate. This suggests a
reaction similar to the cleavage of 2-keto-3-deoxy-6-phosphogluconate to
yield pyruvate and glyceraldehyde-3-phosphate. (The latter reaction,
found in pseudomonads, is a key cleavage in the Entner-Doudoroff
pathway, discussed in Section IV.) Against this idea is the observation
(90) that the proposed lactone of the 2,3-enediol is not metabolized in
Aerobacter and that pentose phosphate enediol is apparently an artifact
of an alkaline environment upon ribulose-5-phosphate. Moreover, 2-ketogluconic acid-6-phosphate, when administered to Aerobacter (89) is
435
enzyme (specific activity = 86) required several hundredfold concentration. The bacterial enzyme employs either pyridine nucleotide as
cofactor, whereas that from mammalian or plant sources tends to be
specific for TPN
2 . The isoelectric point for the enzyme from frog muscle (17) is reported to be at pH 4.8, whereas that from rat blood is
5.85 (17a). Bivalent ions have been shown to activate the enzyme from
yeast and Escherichia colt, but this has not been observed with G-6-P
dehydrogenase from other sources.
3. 6-Phosphogluconate
Dehydrogenase
This enzyme, like glucose-6-phosphate dehydrogenase, has been reported active in many of the systems referred to in the preceding section; this is to be expected since 6-phosphogluconate is the product of
G-6-P oxidation.
6-PGA dehydrogenase is further similar to G-6-P dehydrogenase in
that its purification has been emphasized from microbial systems. Extensive purification of 6-PGA dehydrogenase has been carried out using
yeast (86), Leuconostoc (57), E. colt (52), and Acetobacter suboxydans (66). Plant materials, although mentioned as a source of this
enzyme (25, 51, 87), have not been extensively exploited. The enzyme
from A. sub oxydans has been purified most by means of electrophoresis
on a starch column, so that it is free from G-6-P dehydrogenase (66,
88). Table II contains some comparisons of the enzyme from different
sources.
The oxidation of 6-PGA by TPN involves both a decarboxylation
and a dehydrogenation. As DeLey and Verhofstede (89, 90) have
pointed out, the explanation for the mechanism of this reaction has
shifted back and forth between the speculation that either 2-keto- or
3-keto-6-PGA was formed or that 2, 3-enediol gluconolactone-6-phosphate might also be an intermediate. The idea that 2-keto-6PGA served
as the intermediate was attractive also to Uehara and Akabori (91, 92),
who suggested that 2-keto-6-PGA might undergo an aldolase cleavage
to hydroxypyruvate and glyceraldehyde-3-phosphate. This suggests a
reaction similar to the cleavage of 2-keto-3-deoxy-6-phosphogluconate to
yield pyruvate and glyceraldehyde-3-phosphate. (The latter reaction,
found in pseudomonads, is a key cleavage in the Entner-Doudoroff
pathway, discussed in Section IV.) Against this idea is the observation
(90) that the proposed lactone of the 2,3-enediol is not metabolized in
Aerobacter and that pentose phosphate enediol is apparently an artifact
of an alkaline environment upon ribulose-5-phosphate. Moreover, 2-ketogluconic acid-6-phosphate, when administered to Aerobacter (89) is