1. PENTOSE PHOSPHATE CYCLE
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the crayfish (Orconectes virilis) has been examined, a number of respiratory inhibitors (394) being used, and the respiration was compared
between premolt tissues and intermolt tissues of this crustacean. The
oxygen uptake was inhibited to a greater extent by fluoride and iodoacetate in the premolt tissue than in the intermolt tissue. On this basis, a
hypothesis was forwarded that glycolysis may be the principal metabolic
pathway during premolt whereas the hexose monophosphate shunt may
be the main route during intermolt.
In a book on the biochemistry of insects, Gilmour has documented
the names of many insects in which the reaction systems of the pentose
phosphate cycle occur (395). Thus, the pea aphid, Macrosiphum pisi;
the house fly, Musca domestica; the blow fly Phormia regina; and the
honey bee all contain, in one form or another (i.e., acetone powder,
homogenates, dialyzed extracts of eggs, larvae, and adult insects, etc.),
some signs of the pentose phosphate cycle. More specifically, the house
fly has both TPN-dependent G-6-P DH and 6-PG DH (396) as well as
6-phosphogluconolactonase, phosphopentose isomerase, phosphoketopentose epimerase, transketolase, and transaldolase (397). Both dehydrogenases are localized mainly in the cytoplasm of the flight muscle, but a
significant amount of the G-6-P DH is in the sarcosomes. In general, both
dehydrogenase activities are higher in the male than in the female, and
the site of the highest activities for both of these enzymes is situated in
the abdomen. The presence of TPN-specific dehydrogenase was also
reported in the peach aphid, Myzus persicae (396). In a study of the
bug Triatoma infestans, Agosin et al. demonstrated the existence of
G-6-P DH in cell-free extracts of the thoraxes and legs of larvae, nymphs,
and adults (398). Present also are 6-PG DH and phosphopentose isomerase. All these activities except phosphopentose isomerase are inhibited
by previous exposure of the adult bug to DDT; however, only G-6-P
DH is inhibited when the nymphs are subjected to the same treatment.
From the fact that 6-PG DH is limiting, the pentose phosphate cycle in
carbohydrate metabolism seems to play a minor role. The midgut of the
5th instar larvae of the silkworm, Bombyx mori, also has the potentials
to transform glucose-6-phosphate to ribose plus sedoheptulose, 6-phosphogluconate to ribose, and ribose-5-phosphate to hexose plus sedoheptulose (399). The rate of oxidation of ribose-5-phosphate is lower than
that of glucose-l-phosphate, glucose-6-phosphate, or fructoses-phosphate. Silva et al. (400) applied the method of radiorespirometry (104)
to the study of glucose metabolism in the cockroach Periplaneta americana; they found that in the intact male adult, between 4 and 9% of the
glucose is degraded via the direct oxidative pathway.
As far as is known, the only arthropod of the order Acarina that has
61
the crayfish (Orconectes virilis) has been examined, a number of respiratory inhibitors (394) being used, and the respiration was compared
between premolt tissues and intermolt tissues of this crustacean. The
oxygen uptake was inhibited to a greater extent by fluoride and iodoacetate in the premolt tissue than in the intermolt tissue. On this basis, a
hypothesis was forwarded that glycolysis may be the principal metabolic
pathway during premolt whereas the hexose monophosphate shunt may
be the main route during intermolt.
In a book on the biochemistry of insects, Gilmour has documented
the names of many insects in which the reaction systems of the pentose
phosphate cycle occur (395). Thus, the pea aphid, Macrosiphum pisi;
the house fly, Musca domestica; the blow fly Phormia regina; and the
honey bee all contain, in one form or another (i.e., acetone powder,
homogenates, dialyzed extracts of eggs, larvae, and adult insects, etc.),
some signs of the pentose phosphate cycle. More specifically, the house
fly has both TPN-dependent G-6-P DH and 6-PG DH (396) as well as
6-phosphogluconolactonase, phosphopentose isomerase, phosphoketopentose epimerase, transketolase, and transaldolase (397). Both dehydrogenases are localized mainly in the cytoplasm of the flight muscle, but a
significant amount of the G-6-P DH is in the sarcosomes. In general, both
dehydrogenase activities are higher in the male than in the female, and
the site of the highest activities for both of these enzymes is situated in
the abdomen. The presence of TPN-specific dehydrogenase was also
reported in the peach aphid, Myzus persicae (396). In a study of the
bug Triatoma infestans, Agosin et al. demonstrated the existence of
G-6-P DH in cell-free extracts of the thoraxes and legs of larvae, nymphs,
and adults (398). Present also are 6-PG DH and phosphopentose isomerase. All these activities except phosphopentose isomerase are inhibited
by previous exposure of the adult bug to DDT; however, only G-6-P
DH is inhibited when the nymphs are subjected to the same treatment.
From the fact that 6-PG DH is limiting, the pentose phosphate cycle in
carbohydrate metabolism seems to play a minor role. The midgut of the
5th instar larvae of the silkworm, Bombyx mori, also has the potentials
to transform glucose-6-phosphate to ribose plus sedoheptulose, 6-phosphogluconate to ribose, and ribose-5-phosphate to hexose plus sedoheptulose (399). The rate of oxidation of ribose-5-phosphate is lower than
that of glucose-l-phosphate, glucose-6-phosphate, or fructoses-phosphate. Silva et al. (400) applied the method of radiorespirometry (104)
to the study of glucose metabolism in the cockroach Periplaneta americana; they found that in the intact male adult, between 4 and 9% of the
glucose is degraded via the direct oxidative pathway.
As far as is known, the only arthropod of the order Acarina that has