444
VERNON H. CHELDELIN, CHIH H. WANG, AND TSOO E. KING
set of enzymatic reactions; confusion often resulted from findings in
enzymological studies due to the many enzymes common to several
catabolic pathways. Similar difficulties are also encountered in the
quantitative evaluation of pathway participation inasmuch as the relative rates and extents of concurrent pathways often necessitates detailed
kinetic information on the over-all catabolism of the substrate in question (121).
Radiotracer methods used for quantitative studies of catabolic pathways for carbohydrates can be generally classified into three major
categories:
(1) Methods based on comparison of the specific activities of respiratory C
14 0 2 derived from biological systems catabolizing C
14 specifically
labeled carbohydrates.
(2) Methods based on comparison of the specific activities of C
14 -
labeled metabolic intermediates derived from C
14 specifically labeled
substrates in a given biological system.
(3) The radiorespirometric method (based on the total yield of
C
14 0 2 from C
14 specifically labeled substrates).
1. Methods Based on the Comparison of the Specific Activities
of C
14 0 2 Derived from the Labeled Carbon Atoms of the Substrate
Methods under this category were employed primarily to examine
pathway occurrence and participation in mammalian tissue slices and
plant tissue preparations (14, 15, 122-128a). Evidence collected in
enzymological and other studies has indicated that glycolysis and phosphogluconate cleavage constitute the major mechanisms for hexose dissimilation, and consequently pathway estimations are made on this
basis. Thus, the operation of the Entner-Doudoroff pathway, detected
in microorganisms (114), is ignored since this route is yet to be demonstrated in either animal or plant tissues. It should be pointed out that
inasmuch as a preparation of excised tissue does not generally utilize
all or even a major fraction of substrate glucose in an incubation experiment, it is therefore necessary to use information on the specific activity
of respiratory C
14 0 2 from C
14 specifically labeled substrates for pathway
elucidations.
Qualitatively, the observation of a nonequivalence of the metabolic
behavior of C-l and C-6 of a hexose provides strong evidence for the
operation of phosphogluconate cleavage in the system, since these two
carbon atoms are converted similarly to the methyl carbon atom of
pyruvate in the classic Embden-Meyerhof-Parnas pathway. It is assumed that there exists no preferential dilution of any labeled carbon
atoms of the substrate from endogenous carbon sources. Consequently,
VERNON H. CHELDELIN, CHIH H. WANG, AND TSOO E. KING
set of enzymatic reactions; confusion often resulted from findings in
enzymological studies due to the many enzymes common to several
catabolic pathways. Similar difficulties are also encountered in the
quantitative evaluation of pathway participation inasmuch as the relative rates and extents of concurrent pathways often necessitates detailed
kinetic information on the over-all catabolism of the substrate in question (121).
Radiotracer methods used for quantitative studies of catabolic pathways for carbohydrates can be generally classified into three major
categories:
(1) Methods based on comparison of the specific activities of respiratory C
14 0 2 derived from biological systems catabolizing C
14 specifically
labeled carbohydrates.
(2) Methods based on comparison of the specific activities of C
14 -
labeled metabolic intermediates derived from C
14 specifically labeled
substrates in a given biological system.
(3) The radiorespirometric method (based on the total yield of
C
14 0 2 from C
14 specifically labeled substrates).
1. Methods Based on the Comparison of the Specific Activities
of C
14 0 2 Derived from the Labeled Carbon Atoms of the Substrate
Methods under this category were employed primarily to examine
pathway occurrence and participation in mammalian tissue slices and
plant tissue preparations (14, 15, 122-128a). Evidence collected in
enzymological and other studies has indicated that glycolysis and phosphogluconate cleavage constitute the major mechanisms for hexose dissimilation, and consequently pathway estimations are made on this
basis. Thus, the operation of the Entner-Doudoroff pathway, detected
in microorganisms (114), is ignored since this route is yet to be demonstrated in either animal or plant tissues. It should be pointed out that
inasmuch as a preparation of excised tissue does not generally utilize
all or even a major fraction of substrate glucose in an incubation experiment, it is therefore necessary to use information on the specific activity
of respiratory C
14 0 2 from C
14 specifically labeled substrates for pathway
elucidations.
Qualitatively, the observation of a nonequivalence of the metabolic
behavior of C-l and C-6 of a hexose provides strong evidence for the
operation of phosphogluconate cleavage in the system, since these two
carbon atoms are converted similarly to the methyl carbon atom of
pyruvate in the classic Embden-Meyerhof-Parnas pathway. It is assumed that there exists no preferential dilution of any labeled carbon
atoms of the substrate from endogenous carbon sources. Consequently,