9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
445
the demonstration of a C G :Ci ratio, with respect to C
14 0 2 production
from glucose-6-C
14 and glucose-1-C
14 , respectively, of less than unity
(Ci:C 6 greater than unity) has been used by many authors to indicate
the operation of the HMP pathway in a given biological system (51,
129). However, to estimate quantitatively the participation of each
route, the use of data on the specific activity of C
14 0 2 is subject to
several serious problems. First, as pointed out by Wood (121) the dilution of individual carbon atoms of substrates by endogenous materials
may depend on the pathways followed by the carbon atoms in question.
Several workers (122, 124, 125) have attempted, with the aid of necessary assumptions, to determine the fate of the degradation products of
hexose by the use of C
14 specifically labeled intermediates known to
be derived from glucose in glycolysis or the pentose pathways, as
supplementary test substrates. Some notable examples are the use
of C
14 specifically labeled lactate and specifically labeled gluconate
(122, 125). By the use of specific activity data on C
14 0 2 from glucose1-C
14
, glucose-6-C
14 , or glucose-U-C
14 together with that from the supporting substrates such as lactate, it was possible for these authors to
devise equations for the estimation of pathway participations with respect
to the origin of the respiratory C0 2 . These equations depend upon
the validity of the original assumptions, several of which need experimental verification. Also, the nature of this type of experiment is
such that the data on the specific activity of C
14 0 2 represent the
cumulative information over a limited period of experimental time,
subject to the deterioration of the excised tissue. Moreover, pathway
participation calculated by these equations refers only to the fraction
of respiratory C0 2 derived from an individual pathway, a piece of
information not to be confused with pathway participation in the total
cataholism of glucose inasmuch as there exists no ready correlation
with regard to the amount of C0 2 produced via an individual pathway.
It is obvious that the participation of pathways on the basis of total
glucose catabolized is more meaningful. Equations have been devised
by several authors (126, 127) to estimate pathway participations in
animal tissue making use of the data on the specific activities or yields
of C
14 0 2 from C
14 specifically labeled glucose, lactate, and gluconate
and on the amount of radioactivity incorporated into fatty acids from
the respective labeled substrates.
More serious criticism of the validity of methods of this type was
recently presented by Wood and Katz (130, 131). These authors pointed
out the necessity of taking into consideration the dilution of the administered labeled glucose by the glucose regenerated in the operation
of the pentose cycle reactions. The dilution process is complex in
445
the demonstration of a C G :Ci ratio, with respect to C
14 0 2 production
from glucose-6-C
14 and glucose-1-C
14 , respectively, of less than unity
(Ci:C 6 greater than unity) has been used by many authors to indicate
the operation of the HMP pathway in a given biological system (51,
129). However, to estimate quantitatively the participation of each
route, the use of data on the specific activity of C
14 0 2 is subject to
several serious problems. First, as pointed out by Wood (121) the dilution of individual carbon atoms of substrates by endogenous materials
may depend on the pathways followed by the carbon atoms in question.
Several workers (122, 124, 125) have attempted, with the aid of necessary assumptions, to determine the fate of the degradation products of
hexose by the use of C
14 specifically labeled intermediates known to
be derived from glucose in glycolysis or the pentose pathways, as
supplementary test substrates. Some notable examples are the use
of C
14 specifically labeled lactate and specifically labeled gluconate
(122, 125). By the use of specific activity data on C
14 0 2 from glucose1-C
14
, glucose-6-C
14 , or glucose-U-C
14 together with that from the supporting substrates such as lactate, it was possible for these authors to
devise equations for the estimation of pathway participations with respect
to the origin of the respiratory C0 2 . These equations depend upon
the validity of the original assumptions, several of which need experimental verification. Also, the nature of this type of experiment is
such that the data on the specific activity of C
14 0 2 represent the
cumulative information over a limited period of experimental time,
subject to the deterioration of the excised tissue. Moreover, pathway
participation calculated by these equations refers only to the fraction
of respiratory C0 2 derived from an individual pathway, a piece of
information not to be confused with pathway participation in the total
cataholism of glucose inasmuch as there exists no ready correlation
with regard to the amount of C0 2 produced via an individual pathway.
It is obvious that the participation of pathways on the basis of total
glucose catabolized is more meaningful. Equations have been devised
by several authors (126, 127) to estimate pathway participations in
animal tissue making use of the data on the specific activities or yields
of C
14 0 2 from C
14 specifically labeled glucose, lactate, and gluconate
and on the amount of radioactivity incorporated into fatty acids from
the respective labeled substrates.
More serious criticism of the validity of methods of this type was
recently presented by Wood and Katz (130, 131). These authors pointed
out the necessity of taking into consideration the dilution of the administered labeled glucose by the glucose regenerated in the operation
of the pentose cycle reactions. The dilution process is complex in