16
Ν. G. PON
two pathways (97). Abraham et al. used these same radioactive substrates, and although they found that the so-called C-6:C-1 ratio was
indicative of the operation of the pentose cycle in rat mammary glands,
they used the amount of fatty acid-C
14 recovered from these substrates
to evaluate the relative contribution of the pentose cycle toward the
metabolism of glucose (98).
Other workers varied the type of labeled substrates used; for example,
Heath and Koffler measured the preferential oxidation of the C-l position
of glucose by comparing the utilization of glucose-l-C
14 and glucose-UC
14
(uniformly labeled) (99). Some investigators measured the specific
activities of the lactate, or acetate (100), or pyruvate (100, 101). In
some cases, lactate formed from the labeled glucose was degraded (102,
103). One of the latest methods is the radiorespirometric method of
Wang (104), which, in essence, is a measurement of the rate of C
14 0 2
evolved as a function of incubation time using variously labeled glucose.
It is beyond the scope of this review to discuss the merits of the various
methods. Several critiques have been published (105-107), and a study
of them should allow the reader a more accurate appraisal of these methods. Generally, many investigators now believe that the use of the
"0-6:0-1" ratio alone is not sufficient for determining the relative participation of the pentose phosphate pathway.
Radioactive bicarbonate has been used for studying the pathway of
metabolism of glucose also (108, 109). In these cases, labeled glycogen
was obtained with C
14 being incorporated mainly in carbon atoms 3 and
4. A minor, but reproducible, amount of radioactivity resided in the rest
of the carbon atoms of the glycogen glucose molecule, indicating that a
nonglycolytic pathway is operating during glycogenesis.
The most famous case of the use of C
14 0 2 for studying the pathway
of metabolism is the elucidation of the path of carbon in photosynthesis
(10). These studies date back to about 1948 and have been discussed
earlier in this paper. The point to be reemphasized here is that the labeling pattern was studied as a function of the time of exposure C
14 0 2 .
The labeling pattern, in this case, means both the compounds which
became labeled and the distribution of the label within each compound.
It is interesting to note that Gibbs and Kandier, using this method
with more refined methods of degrading the hexose (with L. mesenteroides), found that the distribution of the label in starch glucose after
short time photosynthesis with C
14 0 2 was asymmetric (110). They
therefore suggested that there may be an alternative pathway of carbon
in photosynthesis. However, Bassham and Calvin have explained the
asymmetric distribution of the radiocarbon via equilibration of the triose
Ν. G. PON
two pathways (97). Abraham et al. used these same radioactive substrates, and although they found that the so-called C-6:C-1 ratio was
indicative of the operation of the pentose cycle in rat mammary glands,
they used the amount of fatty acid-C
14 recovered from these substrates
to evaluate the relative contribution of the pentose cycle toward the
metabolism of glucose (98).
Other workers varied the type of labeled substrates used; for example,
Heath and Koffler measured the preferential oxidation of the C-l position
of glucose by comparing the utilization of glucose-l-C
14 and glucose-UC
14
(uniformly labeled) (99). Some investigators measured the specific
activities of the lactate, or acetate (100), or pyruvate (100, 101). In
some cases, lactate formed from the labeled glucose was degraded (102,
103). One of the latest methods is the radiorespirometric method of
Wang (104), which, in essence, is a measurement of the rate of C
14 0 2
evolved as a function of incubation time using variously labeled glucose.
It is beyond the scope of this review to discuss the merits of the various
methods. Several critiques have been published (105-107), and a study
of them should allow the reader a more accurate appraisal of these methods. Generally, many investigators now believe that the use of the
"0-6:0-1" ratio alone is not sufficient for determining the relative participation of the pentose phosphate pathway.
Radioactive bicarbonate has been used for studying the pathway of
metabolism of glucose also (108, 109). In these cases, labeled glycogen
was obtained with C
14 being incorporated mainly in carbon atoms 3 and
4. A minor, but reproducible, amount of radioactivity resided in the rest
of the carbon atoms of the glycogen glucose molecule, indicating that a
nonglycolytic pathway is operating during glycogenesis.
The most famous case of the use of C
14 0 2 for studying the pathway
of metabolism is the elucidation of the path of carbon in photosynthesis
(10). These studies date back to about 1948 and have been discussed
earlier in this paper. The point to be reemphasized here is that the labeling pattern was studied as a function of the time of exposure C
14 0 2 .
The labeling pattern, in this case, means both the compounds which
became labeled and the distribution of the label within each compound.
It is interesting to note that Gibbs and Kandier, using this method
with more refined methods of degrading the hexose (with L. mesenteroides), found that the distribution of the label in starch glucose after
short time photosynthesis with C
14 0 2 was asymmetric (110). They
therefore suggested that there may be an alternative pathway of carbon
in photosynthesis. However, Bassham and Calvin have explained the
asymmetric distribution of the radiocarbon via equilibration of the triose