6
Ν. G. PON
laboratories isolated salts of pentonic acid phosphates (30, 31). Furthermore, since ribose-5-phosphate was oxidized and fermented more vigorously than other pentose phosphates, Dickens proposed that ribose
phosphate originated from the oxidative decarboxylation of 6-phosphogluconate (31, 32). Meanwhile, Dische showed that hemolyzates of
erythrocytes formed triose phosphate and fructose diphosphate from the
ribose moiety of adenosine (33). Eight years elapsed before work was
continued along this line of pentose metabolism by Schlenk and Waldvogel (34). They found that various nucleosides were broken down more
easily than the corresponding nucleotides and that ribose-5-phosphate
does not require the presence of inorganic phosphate for breakdown (as
compared with an absolute requirement of inorganic phosphate when
adenosine was used as substrate). The enzymes that caused the disappearance of pentoses were found in rat tissues including the liver, kidney,
spleen, brain, blood (35). These data suggested that ribose-5-phosphate
is an intermediate in the formation of triose phosphate and fructose
phosphate. Moreover, these same investigators demonstrated that liver
enzyme plus guanosine yielded glucose-6-phosphate (36). Finally, in
bacterial extracts, Racker found that ribose-5-phosphate is converted to
triose phosphate (37). These facts led to a scheme of reactions in which
glucose phosphate is oxidized to phosphogluconate to pentose phosphate
to triose phosphate, with a C 2 fragment not accounted for. The triose
phosphate is transformed by the already well-known route of isomerization and condensation to fructose diphosphate. This diphosphate ester is
dephosphorylated and is isomerized to glucose phosphate, hence completing the cycle (38). At just about this time, however, Horecker and
his associates found that ribulose-5-phosphate was the product of the
oxidative decarboxylation of 6-phosphogluconate (39-41) and that this
product was further converted to ribose-5-phosphate via the action of an
isomerase (42, 43). [Cohen believed that a 1,2-enediol pentose-5-phosphate is the primary decarboxylation product of 6-phosphogluconate
(38, 44).] In spite of the accumulation of a massive amount of evidence
on the pathway of glucose oxidation, the fate of the two-carbon fragment
remained a thorny unsolved problem. This matter was clarified by the
brilliant research performed mainly by Horecker and Racker and their
co-workers, who discovered in rapid succession transketolase, transaldolase, and a pentose phosphate epimerase (45-49). These enzymes enable
the ribulose phosphate to be converted to xylulose-5-phosphate, the true
substrate for the transketolase reaction. Xylulose phosphate plus ribose
phosphate then yielded sedoheptulose-7-phosphate and glyceraldehyde3-phosphate. These two products, by the action of transaldolase, are
transformed to fructose-6-phosphate and erythrose-4-phosphate. The
Ν. G. PON
laboratories isolated salts of pentonic acid phosphates (30, 31). Furthermore, since ribose-5-phosphate was oxidized and fermented more vigorously than other pentose phosphates, Dickens proposed that ribose
phosphate originated from the oxidative decarboxylation of 6-phosphogluconate (31, 32). Meanwhile, Dische showed that hemolyzates of
erythrocytes formed triose phosphate and fructose diphosphate from the
ribose moiety of adenosine (33). Eight years elapsed before work was
continued along this line of pentose metabolism by Schlenk and Waldvogel (34). They found that various nucleosides were broken down more
easily than the corresponding nucleotides and that ribose-5-phosphate
does not require the presence of inorganic phosphate for breakdown (as
compared with an absolute requirement of inorganic phosphate when
adenosine was used as substrate). The enzymes that caused the disappearance of pentoses were found in rat tissues including the liver, kidney,
spleen, brain, blood (35). These data suggested that ribose-5-phosphate
is an intermediate in the formation of triose phosphate and fructose
phosphate. Moreover, these same investigators demonstrated that liver
enzyme plus guanosine yielded glucose-6-phosphate (36). Finally, in
bacterial extracts, Racker found that ribose-5-phosphate is converted to
triose phosphate (37). These facts led to a scheme of reactions in which
glucose phosphate is oxidized to phosphogluconate to pentose phosphate
to triose phosphate, with a C 2 fragment not accounted for. The triose
phosphate is transformed by the already well-known route of isomerization and condensation to fructose diphosphate. This diphosphate ester is
dephosphorylated and is isomerized to glucose phosphate, hence completing the cycle (38). At just about this time, however, Horecker and
his associates found that ribulose-5-phosphate was the product of the
oxidative decarboxylation of 6-phosphogluconate (39-41) and that this
product was further converted to ribose-5-phosphate via the action of an
isomerase (42, 43). [Cohen believed that a 1,2-enediol pentose-5-phosphate is the primary decarboxylation product of 6-phosphogluconate
(38, 44).] In spite of the accumulation of a massive amount of evidence
on the pathway of glucose oxidation, the fate of the two-carbon fragment
remained a thorny unsolved problem. This matter was clarified by the
brilliant research performed mainly by Horecker and Racker and their
co-workers, who discovered in rapid succession transketolase, transaldolase, and a pentose phosphate epimerase (45-49). These enzymes enable
the ribulose phosphate to be converted to xylulose-5-phosphate, the true
substrate for the transketolase reaction. Xylulose phosphate plus ribose
phosphate then yielded sedoheptulose-7-phosphate and glyceraldehyde3-phosphate. These two products, by the action of transaldolase, are
transformed to fructose-6-phosphate and erythrose-4-phosphate. The