9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
457
(b) At 1 RTU, the C
14 0 2 of the labeled substrate carbon atoms
derived from a relatively direct pathway (such as the conversion of C-l
of glucose to C0 2 via phosphogluconate decarboxylation) represents the
extent of "traffic" traversing this pathway. The radiochemical yields, unlike specific activities of C
14 0 2 , are not seriously affected by dilution of
unlabeled substrate derived from regenerating mechanisms. To illustrate
this, one can again use the conversion of C-l of glucose via phosphogluconate decarboxylation as an example. The C
14 0 2 yield from glucose1-C
14 via phosphogluconate cleavage should remain the same, despite the
fact that the substrate glucose may have been diluted at the glucose-6phosphate stage by fructose-6-phosphate produced from substrate glucose
via the pentose cycle reactions, or by the fructose-l,6-diphosphate produced via the recombination of trioses via the reversed aldolase reaction.
(c) The metabolic nonequivalence observed at 1 RTU with a given
set of labeled carbon atoms of the substrate, i.e., C-l and C-6 of glucose
in the concurrent operation of the glycolytic and the HMP pathway
(Fig. 7C, for example) reflects quantitatively the participation of pathways.
With these basic considerations in mind, one can then derive equations for the estimation of pathway on the basis of the following assumptions:
(1) Consider that glucose may be catabolized in a biological system
via two pathways: one yields two C 3 units and the other yields C0 2 and
a C 5 unit. The former applies either to glycolysis or the EntnerDoudoroff pathway. (There exists no evidence at the present time that
these two pathways are operative concurrently in a given biological system.) The latter type refers to phosphogluconate cleavage.
(2) The preferential conversion of C-l of glucose to C0 2 via phosphogluconate decarboxylation is a rapid and virtually irreversible
process.
(3) The C 3 units formed in either glycolysis or the Entner-Doudoroff
pathway, best represented as pyruvate (Fig. 8), are decarboxylated
promptly. It is understood that a small fraction of pyruvate may participate in C0 2 fixation processes, however, the magnitude of such
processes is likely to be insignificant.
(4) The C 3 units (i.e., trioses and pyruvate) formed in glycolysis or
the Entner-Doudoroff pathway are virtually equivalent to each other
at the pyruvate stage with respect to further metabolic reactions.
(5) The formation of hexose by way of a recombination of trioses is
insignificant in magnitude.
(6) The randomization of the hexose skeleton via transketolase ex-
457
(b) At 1 RTU, the C
14 0 2 of the labeled substrate carbon atoms
derived from a relatively direct pathway (such as the conversion of C-l
of glucose to C0 2 via phosphogluconate decarboxylation) represents the
extent of "traffic" traversing this pathway. The radiochemical yields, unlike specific activities of C
14 0 2 , are not seriously affected by dilution of
unlabeled substrate derived from regenerating mechanisms. To illustrate
this, one can again use the conversion of C-l of glucose via phosphogluconate decarboxylation as an example. The C
14 0 2 yield from glucose1-C
14 via phosphogluconate cleavage should remain the same, despite the
fact that the substrate glucose may have been diluted at the glucose-6phosphate stage by fructose-6-phosphate produced from substrate glucose
via the pentose cycle reactions, or by the fructose-l,6-diphosphate produced via the recombination of trioses via the reversed aldolase reaction.
(c) The metabolic nonequivalence observed at 1 RTU with a given
set of labeled carbon atoms of the substrate, i.e., C-l and C-6 of glucose
in the concurrent operation of the glycolytic and the HMP pathway
(Fig. 7C, for example) reflects quantitatively the participation of pathways.
With these basic considerations in mind, one can then derive equations for the estimation of pathway on the basis of the following assumptions:
(1) Consider that glucose may be catabolized in a biological system
via two pathways: one yields two C 3 units and the other yields C0 2 and
a C 5 unit. The former applies either to glycolysis or the EntnerDoudoroff pathway. (There exists no evidence at the present time that
these two pathways are operative concurrently in a given biological system.) The latter type refers to phosphogluconate cleavage.
(2) The preferential conversion of C-l of glucose to C0 2 via phosphogluconate decarboxylation is a rapid and virtually irreversible
process.
(3) The C 3 units formed in either glycolysis or the Entner-Doudoroff
pathway, best represented as pyruvate (Fig. 8), are decarboxylated
promptly. It is understood that a small fraction of pyruvate may participate in C0 2 fixation processes, however, the magnitude of such
processes is likely to be insignificant.
(4) The C 3 units (i.e., trioses and pyruvate) formed in glycolysis or
the Entner-Doudoroff pathway are virtually equivalent to each other
at the pyruvate stage with respect to further metabolic reactions.
(5) The formation of hexose by way of a recombination of trioses is
insignificant in magnitude.
(6) The randomization of the hexose skeleton via transketolase ex-