60
Ν. G. PON
ing to the same family is the steelhead salmon, Salmo gairdnerii, and it
is not too surprising to find the operation of the pentose phosphate cycle
in this fish (389). However, intramuscular or intravenous injection of
labeled 6-phosphogluconate showed that this ester is only slowly
metabolized via the hexose monophosphate shunt. The comparative
assessment of the relative participation of the two major pathways is
yet to be made. Glucose-l-C
14 or glucose-6-C
14 administered to carps
gave essentially the same recovery of respired C0 2 (390). These data
along with other data on the equal incorporation of C
14 from these two
substrates into liver fat and protein-glutamic acid and alanine indicate
that the oxidation of glucose in the carp is primarily via the EMP
scheme and to a small extent by the shunt mechanism. In addition, the
levels of both G-6-P DH and 6-PG DH in the carp liver are low. On
the other hand, the carp tail muscle oxidizes glucose to yield a C-6:C-l
ratio of 0.07 in the presence of a bicarbonate buffer (384). Tarr has
isolated, partially purified, and characterized phosphoribose isomerase
and epimerase from the muscles of lingcod, Ophiodon elongatus
(391).
The intermediary metabolism by tissues of the electric fish Electrophorus
electricus, is generally the same as that of the caiman or the carp tail
muscle (384). Except for the brain of the electric fish, the ratios of radiochemical yields of C0 2 of the other tissues, such as the main organ, the
Sachs organ, the intact electroplax, and the skin, average ca. 0.2.
VII. Arthropods
Carbohydrate metabolism in this phylum has been investigated
mainly in three classes of animals: the crustacean, the insect, and the
mite. Of the crustaceans, only those of the subclass belonging to the
crab, lobster, and crayfish were studied. In the crabs Cancer magister
(392) and Hemigrapsus nudus (393) there is no evidence that the
pentose phosphate cycle is utilized to oxidize glucose to C0 2 , although
the former is quite capable of yielding labeled C0 2 from maltose-C
14
,
and the latter can utilize glucose as an oxidative substrate in all stages
of the intermolt cycle. The stimulation of oxygen consumption in lobster
(Homarus americanus) hepatopancreas homogenates by pentose phosphate cycle intermediates has already meen mentioned (cf. Section V, C)
(66). In addition, when acetate-l-C
14 was administered to tissue slices,
in the presence and absence of fluoroacetate, the label was incorporated
mostly into the carbon-3 and -4 positions of the glycogen glucose; however, there was some randomization of the label into the other four carbon atoms. From this fact, a 5-10% participation of the pentose phosphate
cycle was estimated. The enzymes of both glycolysis and the shunt were
found in the extramitochondrial fraction. Carbohydrate metabolism in
Ν. G. PON
ing to the same family is the steelhead salmon, Salmo gairdnerii, and it
is not too surprising to find the operation of the pentose phosphate cycle
in this fish (389). However, intramuscular or intravenous injection of
labeled 6-phosphogluconate showed that this ester is only slowly
metabolized via the hexose monophosphate shunt. The comparative
assessment of the relative participation of the two major pathways is
yet to be made. Glucose-l-C
14 or glucose-6-C
14 administered to carps
gave essentially the same recovery of respired C0 2 (390). These data
along with other data on the equal incorporation of C
14 from these two
substrates into liver fat and protein-glutamic acid and alanine indicate
that the oxidation of glucose in the carp is primarily via the EMP
scheme and to a small extent by the shunt mechanism. In addition, the
levels of both G-6-P DH and 6-PG DH in the carp liver are low. On
the other hand, the carp tail muscle oxidizes glucose to yield a C-6:C-l
ratio of 0.07 in the presence of a bicarbonate buffer (384). Tarr has
isolated, partially purified, and characterized phosphoribose isomerase
and epimerase from the muscles of lingcod, Ophiodon elongatus
(391).
The intermediary metabolism by tissues of the electric fish Electrophorus
electricus, is generally the same as that of the caiman or the carp tail
muscle (384). Except for the brain of the electric fish, the ratios of radiochemical yields of C0 2 of the other tissues, such as the main organ, the
Sachs organ, the intact electroplax, and the skin, average ca. 0.2.
VII. Arthropods
Carbohydrate metabolism in this phylum has been investigated
mainly in three classes of animals: the crustacean, the insect, and the
mite. Of the crustaceans, only those of the subclass belonging to the
crab, lobster, and crayfish were studied. In the crabs Cancer magister
(392) and Hemigrapsus nudus (393) there is no evidence that the
pentose phosphate cycle is utilized to oxidize glucose to C0 2 , although
the former is quite capable of yielding labeled C0 2 from maltose-C
14
,
and the latter can utilize glucose as an oxidative substrate in all stages
of the intermolt cycle. The stimulation of oxygen consumption in lobster
(Homarus americanus) hepatopancreas homogenates by pentose phosphate cycle intermediates has already meen mentioned (cf. Section V, C)
(66). In addition, when acetate-l-C
14 was administered to tissue slices,
in the presence and absence of fluoroacetate, the label was incorporated
mostly into the carbon-3 and -4 positions of the glycogen glucose; however, there was some randomization of the label into the other four carbon atoms. From this fact, a 5-10% participation of the pentose phosphate
cycle was estimated. The enzymes of both glycolysis and the shunt were
found in the extramitochondrial fraction. Carbohydrate metabolism in