1. PENTOSE PHOSPHATE CYCLE
59
C. REPTILES, AMPHIBIANS, AND FISH
The only data, apparently, that exist concerning the occurrence of the
pentose phosphate cycle for reptiles, are those studies on the cayman (or
caiman) (884). From glucose-l-C
14 and glucose-6-C
14
, the radiochemical
yield of respiratory C0 2 gave C-6:C-l ratios of 0.08 and 0.40 for the tail
muscle and the intercostal muscle, respectively, in Krebs-Ringer bicarbonate buffer. In Krebs-Ringer phosphate buffer, the tail muscle gave a
higher C-6:C-l ratio and the oxygen consumption was insignificantly
low. Under the same conditions, rat diaphragm which has been shown
repeatedly to catabolize glucose exclusively by the EMP scheme, has a
C-6:C-l ratio of about 1.2 either in Krebs-Ringer bicarbonate or in
Krebs-Ringer phosphate medium. Possibly, the carboxylation of pyruvate
to form malate with the concomitant production of TPN
+ may account
for the enhanced activity of the oxidative portion of the pentose phosphate cycle in the presence of the bicarbonate buffer.
In the amphibian class, G-6-P DH and 6-PG DH activity were found
in frog heart (385). The frog liver as well as the liver of the tadpole
(386) were tested for G-6-P DH also. In this case, the adult frog liver
has a lower activity of G-6-P DH than the tadpole liver. Metamorphosis,
induced by thyroxine, caused a decrease in the dehydrogenase activity to
75-80% of the premetamorphic value. It is not known whether the fall in
enzymatic activity is due to a change in the amount of enzyme, the
removal or formation of inhibitor, or a change in the stability of the
enzyme.
At least a half dozen different types of fishes have been examined
with respect to glucose metabolism via the pentose phosphate cycle. By
studying the incorporation of label from variously labeled glucose into
C0 2 and fats under aerobic and anaerobic conditions, Hochachka concluded that the results can be explained by the sole operation of the
EMP pathway (387) in the muscles of the trout Sahelinus fontinalis. In
the goldfish, glucose is oxidized to C0 2 with a C-6:C-l ratio of 0.9 under
pure oxygen in vivo; however, there was evidence of preferential incorporation of carbon from glucose-6-C
14 into liver and muscle glycogen as
well as fat. Further studies of the effect of temperature acclimation on
the pathways of glucose metabolism in the trout suggested that the predominant pathway of glucose metabolism in the 15° acclimated trout is
the EMP route whereas in the 4° acclimated trout, there is an increased
participation of the pentose phosphate cycle along with a higher rate of
fatty acid synthesis (388). The latter findings confirmed that during
anoxia, liver slices have an active pentose phosphate cycle (387). Belong-
59
C. REPTILES, AMPHIBIANS, AND FISH
The only data, apparently, that exist concerning the occurrence of the
pentose phosphate cycle for reptiles, are those studies on the cayman (or
caiman) (884). From glucose-l-C
14 and glucose-6-C
14
, the radiochemical
yield of respiratory C0 2 gave C-6:C-l ratios of 0.08 and 0.40 for the tail
muscle and the intercostal muscle, respectively, in Krebs-Ringer bicarbonate buffer. In Krebs-Ringer phosphate buffer, the tail muscle gave a
higher C-6:C-l ratio and the oxygen consumption was insignificantly
low. Under the same conditions, rat diaphragm which has been shown
repeatedly to catabolize glucose exclusively by the EMP scheme, has a
C-6:C-l ratio of about 1.2 either in Krebs-Ringer bicarbonate or in
Krebs-Ringer phosphate medium. Possibly, the carboxylation of pyruvate
to form malate with the concomitant production of TPN
+ may account
for the enhanced activity of the oxidative portion of the pentose phosphate cycle in the presence of the bicarbonate buffer.
In the amphibian class, G-6-P DH and 6-PG DH activity were found
in frog heart (385). The frog liver as well as the liver of the tadpole
(386) were tested for G-6-P DH also. In this case, the adult frog liver
has a lower activity of G-6-P DH than the tadpole liver. Metamorphosis,
induced by thyroxine, caused a decrease in the dehydrogenase activity to
75-80% of the premetamorphic value. It is not known whether the fall in
enzymatic activity is due to a change in the amount of enzyme, the
removal or formation of inhibitor, or a change in the stability of the
enzyme.
At least a half dozen different types of fishes have been examined
with respect to glucose metabolism via the pentose phosphate cycle. By
studying the incorporation of label from variously labeled glucose into
C0 2 and fats under aerobic and anaerobic conditions, Hochachka concluded that the results can be explained by the sole operation of the
EMP pathway (387) in the muscles of the trout Sahelinus fontinalis. In
the goldfish, glucose is oxidized to C0 2 with a C-6:C-l ratio of 0.9 under
pure oxygen in vivo; however, there was evidence of preferential incorporation of carbon from glucose-6-C
14 into liver and muscle glycogen as
well as fat. Further studies of the effect of temperature acclimation on
the pathways of glucose metabolism in the trout suggested that the predominant pathway of glucose metabolism in the 15° acclimated trout is
the EMP route whereas in the 4° acclimated trout, there is an increased
participation of the pentose phosphate cycle along with a higher rate of
fatty acid synthesis (388). The latter findings confirmed that during
anoxia, liver slices have an active pentose phosphate cycle (387). Belong-