62
Ν. G. PON
been analyzed for enzymes of the pentose phosphate cycle is the twospotted spider mite, Tetranychus telarius L. (86). Homogenates of this
mite contain active G-6-P DH, 6-PG DH, pentose phosphate isomerase,
phosphoketopentose epimerase, transketolase, and transaldolase.
VIII. Segmented Worms
Cohen studied the ribose-forming pathway of the polychaete worm
Chaetopterus, during its early embryonic stage (401). He found that
there exist sufficient G-6-P DH and 6-PG DH in the unfertilized ovum to
synthesize all the ribose in the ribonucleic acid in an interval of 10 hours
at 25°. In fact, when the fertilized eggs divide and differentiate into a
swimming ciliated form, both the ribonucleic acid content and the activities of the dehydrogenases in the embryonic annelid are constant during
this interval of time. Other annelids possessing G-6-P DH and 6-PG DH
activities are the earth worms Lumbricus terrestris and Tubifex tubifex
(402).
IX. Mollusks
Examples of this phylum that have been studied with respect to pentose phosphate pathway activity are the cephalopod Loligo pealei and
the gastropod Lymnaea stagnalis. In both cases, the presence of pentose
phosphate cycle enzymes were demonstrated. Thus the sheath and the
axoplasm of the giant axon of the squid contain both G-6-P DH and
6-PG DH (403). In the 7-day-old embryo of the snail Lymnaea, practically all the enzymes of the shunt were found (404); however, the 6-PG
DH activity is very low. From this last fact, the author speculated that
the contribution of the direct oxidative pathway to glucose metabolism
must be minor, although the high G-6-P DH activity speaks for its role
in TPNH-linked synthetic processes.*
X. Echinoderms
A representative of this phylum is the sea urchin. One of the early
works already mentioned in Section V, A of this review (60) yielded the
first suggestive evidence for the existence of the shunt mechanism of sea
urchin eggs. More substantial proof for the occurrence of this route of
metabolism was obtained by Lindberg (405) in the gametes of Echinocardium cordatum, by Krahl et al. (406) in the eggs of Arbacia punctulata, and by Bäckström (407) in the eggs of Paracentrotus lividus and
Psammechinus miliaris. In all these examples, G-6-P DH was shown to
be present, and in Arbacia the presence of 6-PG DH was also estab* Recently, another aquatic snail, Physa halei Lea, has been shown to contain
TPN linked G-6-P DH and 6-PG DH (404a).
Ν. G. PON
been analyzed for enzymes of the pentose phosphate cycle is the twospotted spider mite, Tetranychus telarius L. (86). Homogenates of this
mite contain active G-6-P DH, 6-PG DH, pentose phosphate isomerase,
phosphoketopentose epimerase, transketolase, and transaldolase.
VIII. Segmented Worms
Cohen studied the ribose-forming pathway of the polychaete worm
Chaetopterus, during its early embryonic stage (401). He found that
there exist sufficient G-6-P DH and 6-PG DH in the unfertilized ovum to
synthesize all the ribose in the ribonucleic acid in an interval of 10 hours
at 25°. In fact, when the fertilized eggs divide and differentiate into a
swimming ciliated form, both the ribonucleic acid content and the activities of the dehydrogenases in the embryonic annelid are constant during
this interval of time. Other annelids possessing G-6-P DH and 6-PG DH
activities are the earth worms Lumbricus terrestris and Tubifex tubifex
(402).
IX. Mollusks
Examples of this phylum that have been studied with respect to pentose phosphate pathway activity are the cephalopod Loligo pealei and
the gastropod Lymnaea stagnalis. In both cases, the presence of pentose
phosphate cycle enzymes were demonstrated. Thus the sheath and the
axoplasm of the giant axon of the squid contain both G-6-P DH and
6-PG DH (403). In the 7-day-old embryo of the snail Lymnaea, practically all the enzymes of the shunt were found (404); however, the 6-PG
DH activity is very low. From this last fact, the author speculated that
the contribution of the direct oxidative pathway to glucose metabolism
must be minor, although the high G-6-P DH activity speaks for its role
in TPNH-linked synthetic processes.*
X. Echinoderms
A representative of this phylum is the sea urchin. One of the early
works already mentioned in Section V, A of this review (60) yielded the
first suggestive evidence for the existence of the shunt mechanism of sea
urchin eggs. More substantial proof for the occurrence of this route of
metabolism was obtained by Lindberg (405) in the gametes of Echinocardium cordatum, by Krahl et al. (406) in the eggs of Arbacia punctulata, and by Bäckström (407) in the eggs of Paracentrotus lividus and
Psammechinus miliaris. In all these examples, G-6-P DH was shown to
be present, and in Arbacia the presence of 6-PG DH was also estab* Recently, another aquatic snail, Physa halei Lea, has been shown to contain
TPN linked G-6-P DH and 6-PG DH (404a).