1. PENTOSE PHOSPHATE CYCLE
7
fructose phosphate is isomerized to glucose-6-phosphate, and the tetrose
phosphate served as an acceptor for another C 2 fragment from the action
of transketolase on a second xylulose phosphate molecule. In the latter
case, fructose phosphate is again formed.
What happens to the triose phosphate in the meantime? According
to Fig. 1, this intermediate would accumulate with time if it is not
diverted into other pathways. Actually, there is ample evidence that this
compound is metabolized via the familiar "triose phosphate enzymes"
such as aldolase, triose phosphate isomerase (in the reverse direction of
the Embden-Meyerhof-Parnas scheme) or triose phosphate dehydrogenase, phosphoglycerate mutase, enolase, and pyruvate kinase in the
forward direction (5).
The work described above was concerned with the pentose phosphate
cycle in vitro; i.e., experiments with homogenates and partially purified
enzymes, but the question always arises whether the sequences catalyzed
by the in vitro enzyme systems actually occur in the whole cell. In 1951,
Cohen showed that Escherichia coli adapted to grow in gluconate, when
fed gluconate-l-C
14
, yielded a 100% recovery of carbon atom 1 (C-l) as
C0 2 for growth aerobically and for oxidation without growth (44). He
concluded that gluconate was utilized almost completely via the oxidative pathway. A classic example of the in vivo operation of the oxidative
pathway of glucose metabolism was demonstrated in the heterolactic
fermentation of glucose-l-C
14 and glucose-3,4-C
14 by Leuconostoc mesenteroides (50).
B. THE REDUCTIVE PENTOSE PHOSPHATE PATHWAY
While the studies on the oxidative pentose phosphate cycle were
being executed, radioactive bicarbonate was used for determining the
path of carbon in photosynthesis. Benson and Calvin described the first
application of C
14 0 2 to the study of the effect of preillumination on the
dark assimilation of C0 2 in Chlorella pyrenoidosa (51). They obtained
data which suggested that phosphoglycerate (PGA) was the first stable
product of C0 2 fixation. These results were confirmed by another group
(52). With the advent of paper chromatography coupled with the detection of radioactive material by radioautography, thus permitting rapid
separation and identification of many compounds simultaneously, more
detailed kinetic studies were possible. These studies showed that as
Scenedesmus obliquus was exposed to C
14 0 2 in the light for shorter and
shorter times, the labeled phosphoglycerate spot on the chromatogram
became more and more dominant until for 5 seconds' photosynthesis,
about 90% of the tracer was found in this compound with about 90% of its
label in the carboxyl group (53). Among the earliest sugar phosphates
7
fructose phosphate is isomerized to glucose-6-phosphate, and the tetrose
phosphate served as an acceptor for another C 2 fragment from the action
of transketolase on a second xylulose phosphate molecule. In the latter
case, fructose phosphate is again formed.
What happens to the triose phosphate in the meantime? According
to Fig. 1, this intermediate would accumulate with time if it is not
diverted into other pathways. Actually, there is ample evidence that this
compound is metabolized via the familiar "triose phosphate enzymes"
such as aldolase, triose phosphate isomerase (in the reverse direction of
the Embden-Meyerhof-Parnas scheme) or triose phosphate dehydrogenase, phosphoglycerate mutase, enolase, and pyruvate kinase in the
forward direction (5).
The work described above was concerned with the pentose phosphate
cycle in vitro; i.e., experiments with homogenates and partially purified
enzymes, but the question always arises whether the sequences catalyzed
by the in vitro enzyme systems actually occur in the whole cell. In 1951,
Cohen showed that Escherichia coli adapted to grow in gluconate, when
fed gluconate-l-C
14
, yielded a 100% recovery of carbon atom 1 (C-l) as
C0 2 for growth aerobically and for oxidation without growth (44). He
concluded that gluconate was utilized almost completely via the oxidative pathway. A classic example of the in vivo operation of the oxidative
pathway of glucose metabolism was demonstrated in the heterolactic
fermentation of glucose-l-C
14 and glucose-3,4-C
14 by Leuconostoc mesenteroides (50).
B. THE REDUCTIVE PENTOSE PHOSPHATE PATHWAY
While the studies on the oxidative pentose phosphate cycle were
being executed, radioactive bicarbonate was used for determining the
path of carbon in photosynthesis. Benson and Calvin described the first
application of C
14 0 2 to the study of the effect of preillumination on the
dark assimilation of C0 2 in Chlorella pyrenoidosa (51). They obtained
data which suggested that phosphoglycerate (PGA) was the first stable
product of C0 2 fixation. These results were confirmed by another group
(52). With the advent of paper chromatography coupled with the detection of radioactive material by radioautography, thus permitting rapid
separation and identification of many compounds simultaneously, more
detailed kinetic studies were possible. These studies showed that as
Scenedesmus obliquus was exposed to C
14 0 2 in the light for shorter and
shorter times, the labeled phosphoglycerate spot on the chromatogram
became more and more dominant until for 5 seconds' photosynthesis,
about 90% of the tracer was found in this compound with about 90% of its
label in the carboxyl group (53). Among the earliest sugar phosphates