9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
447
C-l of glucose is transformed into C-2 of acetate via glycolysis but is
not incorporated into acetate at all by way of the pentose pathway,
whereas C-6 of glucose can be transformed to C-2 of acetate via either
route; the acetate or other intermediate can be isolated in sizable quantity. This is easily done in microbial or plant metabolism but is more
difficult in many animal experiments, particularly when tissue preparations are used, since accumulation of intermediates is generally not
realized. This difficulty is sometimes circumvented by the addition of
unlabeled acetate or other intermediates to the incubation system to
serve as trapping agent and to obtain a sufficient amount of labeled
intermediate for analysis; however, the imposition of a nonphysiological
condition to the experiment may upset the normal metabolism of the
system under study.
The problem of the reduction of the specific activity of the metabolic
product by nonlabeled diluent derived from endogenous material is
another serious limitation of this method. However, the problem is
handled ingeniously (132) through the use of glucose-U-C
14 as sole substrate in a concurrent experiment. The specific activity of the metabolic
product derived from glucose-U-C
14 is then taken as the standard for
the necessary correction for endogenous dilution.
It should be emphasized that the method as devised by Blumenthal,
Lewis and Weinhouse (132) aims at the estimation of pathway participation with respect to the glucose administered. No consideration has
been given to the dilution effect of regenerated fructose-6-phosphate upon the substrate glucose. Thus one of the conditions set forth by the
authors requires that the pentose phosphate formed in the HMP pathway shall not cause the regeneration of fructose-6-phosphate. The extent
of this dilution, as explained later in the text, is not likely to be great
enough to affect the validity of the findings in an experiment using
glucose-1-C
14 , particularly if the operation of the pentose pathway is
relatively minor in the over-all glucose catabolism.
More recently, the dilution of the administered labeled glucose by
the hexose phosphates regenerated via the pentose cycle has been
examined more closely by Dawes and Holms (135-137) in Sarcina lutea.
These authors compared the fraction of conversion of the initial substrate, glucose-1-C
14 , glucose-2-C
14 , or glucose-6-C
14 to the metabolic
product pyruvate as an indicator of glucose catabolism in the organism.
The dilution of the labeled glucose by regenerated hexose phosphates
was considered by assuming that the fructose-6-phosphate produced
via the pentose cycle could be catabolized further by both glycolysis
and the pentose cycle, in the same proportion as could the substrate
glucose. Equations were devised to calculate the theoretical radio-
447
C-l of glucose is transformed into C-2 of acetate via glycolysis but is
not incorporated into acetate at all by way of the pentose pathway,
whereas C-6 of glucose can be transformed to C-2 of acetate via either
route; the acetate or other intermediate can be isolated in sizable quantity. This is easily done in microbial or plant metabolism but is more
difficult in many animal experiments, particularly when tissue preparations are used, since accumulation of intermediates is generally not
realized. This difficulty is sometimes circumvented by the addition of
unlabeled acetate or other intermediates to the incubation system to
serve as trapping agent and to obtain a sufficient amount of labeled
intermediate for analysis; however, the imposition of a nonphysiological
condition to the experiment may upset the normal metabolism of the
system under study.
The problem of the reduction of the specific activity of the metabolic
product by nonlabeled diluent derived from endogenous material is
another serious limitation of this method. However, the problem is
handled ingeniously (132) through the use of glucose-U-C
14 as sole substrate in a concurrent experiment. The specific activity of the metabolic
product derived from glucose-U-C
14 is then taken as the standard for
the necessary correction for endogenous dilution.
It should be emphasized that the method as devised by Blumenthal,
Lewis and Weinhouse (132) aims at the estimation of pathway participation with respect to the glucose administered. No consideration has
been given to the dilution effect of regenerated fructose-6-phosphate upon the substrate glucose. Thus one of the conditions set forth by the
authors requires that the pentose phosphate formed in the HMP pathway shall not cause the regeneration of fructose-6-phosphate. The extent
of this dilution, as explained later in the text, is not likely to be great
enough to affect the validity of the findings in an experiment using
glucose-1-C
14 , particularly if the operation of the pentose pathway is
relatively minor in the over-all glucose catabolism.
More recently, the dilution of the administered labeled glucose by
the hexose phosphates regenerated via the pentose cycle has been
examined more closely by Dawes and Holms (135-137) in Sarcina lutea.
These authors compared the fraction of conversion of the initial substrate, glucose-1-C
14 , glucose-2-C
14 , or glucose-6-C
14 to the metabolic
product pyruvate as an indicator of glucose catabolism in the organism.
The dilution of the labeled glucose by regenerated hexose phosphates
was considered by assuming that the fructose-6-phosphate produced
via the pentose cycle could be catabolized further by both glycolysis
and the pentose cycle, in the same proportion as could the substrate
glucose. Equations were devised to calculate the theoretical radio-