9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
449
catabolic pathways. Information collected in an experiment of this type
is expressed as percentage radiochemical recovery of the labeled carbon
atom in the respiratory C0 2 per unit time, and the kinetics of the process are automatically displayed when one plots the interval recoveries
versus time. The cumulative C
14 0 2 recovery, and the radioactivity in
the medium of incubation or fermentation products from microorganisms
as well as radioactivity in the organisms after incubation, also provide
important information on the over-all assimilation of the administered
substrate (140). Such patterns are also very useful in the elucidation
of catabolic mechanisms.
It should be noted that the data based on interval C
14 0 2 recoveries
collected in the radiorespirometric experiment are related to the data on
specific activity of C
14 0 2 described under method no. 1 since C0 2 yields
from organisms in concurrent experiments under the same conditions
tend to be alike. The advantage in the use of the interval C
14 0 2 yields
lies in the fact that they are not affected by endogenous unlabeled
material or other unlabeled carbon sources in the incubation medium,
thus permitting one to trace the fate of a given carbon atom of a
particular substrate in cellular respiratory functions. This is an important point if it is realized that the conventional respirometric method
(e.g., with Warburg flasks) does not provide information on the respiratory activity of a given substrate, e.g., glucose, in the presence of
other carbon sources such as yeast extract or peptone.
The kinetic feature of the radiorespirometric method permits one to
examine the progress of an experiment if a rapid counting technique,
such as liquid scintillation counting, is used for the assay of the radioactivity in the respiratory C0 2 (141). With proper experimental design,
it is therefore possible to trace the complete biological fate of a given
substrate, i.e., complete utilization of the administered substrate with
respect to all catabolic pathways. This is important since the over-all
catabolic picture so obtained will not suffer greatly from the variation
in reaction rates when two or more concurrent catabolic sequences are
under observation.
Illustrations of the data analysis used in the radiorespirometric method
are given in Fig. 7, parts A, B, and C. Three patterns of glucose
catabolism are represented by the three organisms chosen: Zymomonas
mobilis, Acetobacter suboxydans, and Bacillus subtilis. Each represents
a specific type, and the patterns of C
14 0 2 evolution are characteristic of
each type, as follows:
(a) The first of these characteristic responses is given by Z. mobilis
(142). This organism, which reportedly possesses no Krebs cycle (143),
shows a radiorespirometric pattern, given in Fig. 7A, in which C-1 and
449
catabolic pathways. Information collected in an experiment of this type
is expressed as percentage radiochemical recovery of the labeled carbon
atom in the respiratory C0 2 per unit time, and the kinetics of the process are automatically displayed when one plots the interval recoveries
versus time. The cumulative C
14 0 2 recovery, and the radioactivity in
the medium of incubation or fermentation products from microorganisms
as well as radioactivity in the organisms after incubation, also provide
important information on the over-all assimilation of the administered
substrate (140). Such patterns are also very useful in the elucidation
of catabolic mechanisms.
It should be noted that the data based on interval C
14 0 2 recoveries
collected in the radiorespirometric experiment are related to the data on
specific activity of C
14 0 2 described under method no. 1 since C0 2 yields
from organisms in concurrent experiments under the same conditions
tend to be alike. The advantage in the use of the interval C
14 0 2 yields
lies in the fact that they are not affected by endogenous unlabeled
material or other unlabeled carbon sources in the incubation medium,
thus permitting one to trace the fate of a given carbon atom of a
particular substrate in cellular respiratory functions. This is an important point if it is realized that the conventional respirometric method
(e.g., with Warburg flasks) does not provide information on the respiratory activity of a given substrate, e.g., glucose, in the presence of
other carbon sources such as yeast extract or peptone.
The kinetic feature of the radiorespirometric method permits one to
examine the progress of an experiment if a rapid counting technique,
such as liquid scintillation counting, is used for the assay of the radioactivity in the respiratory C0 2 (141). With proper experimental design,
it is therefore possible to trace the complete biological fate of a given
substrate, i.e., complete utilization of the administered substrate with
respect to all catabolic pathways. This is important since the over-all
catabolic picture so obtained will not suffer greatly from the variation
in reaction rates when two or more concurrent catabolic sequences are
under observation.
Illustrations of the data analysis used in the radiorespirometric method
are given in Fig. 7, parts A, B, and C. Three patterns of glucose
catabolism are represented by the three organisms chosen: Zymomonas
mobilis, Acetobacter suboxydans, and Bacillus subtilis. Each represents
a specific type, and the patterns of C
14 0 2 evolution are characteristic of
each type, as follows:
(a) The first of these characteristic responses is given by Z. mobilis
(142). This organism, which reportedly possesses no Krebs cycle (143),
shows a radiorespirometric pattern, given in Fig. 7A, in which C-1 and