70
Ν. G. PON
mutase and TPN-linked triose phosphate dehydrogenase, have been
demonstrated in cell-free extracts of the green Euglena (425). When the
Euglena is grown in the dark, however, or when it is bleached with
streptomyocin, the extracts no longer contain these enzymes. On the
other hand, the naturally colorless euglenoid Astasia possesses carboxydismutase activity, but no TPN-specific triose phosphate dehydrogenase
activity.
With regard to the blue-green algae, at least in two types, Anacystis
and Nostoc, the TPN-dependent glyceraldehyde-3-phosphate dehydrogenase was found in cell-free extracts (83). Furthermore, carboxydismutase was detected in cell-free extracts of Anacystis nidulans (70).
These indications, considered along with the typical photosynthetic C0 2
fixation pattern of compounds obtained in Nostoc muscorum
(451),
would lead one to believe that the carbon reduction cycle as shown in
Fig. 2 is operative in the blue-green algae. Fewson et al, however, have
shown without any doubt that fructose-1,6-diphosphate aldolase is absent
in four such algae: Anacystis, Anabaena, Nostoc, and Tolypothrix (83).
Thus, a controversial point is raised concerning the validity of the reductive pentose phosphate cycle or the carbon reduction cycle. Peterkofsky
and Racker pointed out that certain enzymes of this cycle are deficient
even in green algae and leaves so that the cycle cannot support the
rate of photosynthesis by the intact cell (88). These and other evidences,
such as the asymmetric labeling of glucose from C
14 0 2 fixation in the
light and the sensitivity toward iodoacetate inhibition (456), are not
compatible with the operation of the reductive pentose phosphate cycle
as such. Hence, an alternate pathway was proposed that did not involve
the hydrolytic cleavage of the presumed six-carbon intermediate of the
carboxydismutase reaction, and did not involve the recondensation of
two molecules of triose phosphates (456). This whole question has been
discussed in great detail by Wassink in his review article on photosynthesis (457). Before leaving this problem altogether, it seems to this
reviewer that another possibility exists for the condensation of threecarbon compounds to form hexoses via the reversal of the EntnerDoudoroff pathway since it has been recently shown that the 2-keto3-deoxy-6-phosphogluconate aldolase is reversible (458). So far, however, no such enzyme has been detected in algae.
Just as in the case of the autotrophic bacteria, carboxydismutase in
algae is apparently an inducible enzyme. In Table XVI, Chlorella varU
egata as an autotroph is listed as containing reductive pentose phosphate
cycle activity. The heterotrophically grown C. variegata, on the other
hand, possessing no chlorophyll, also has no carboxydismutase activity
Ν. G. PON
mutase and TPN-linked triose phosphate dehydrogenase, have been
demonstrated in cell-free extracts of the green Euglena (425). When the
Euglena is grown in the dark, however, or when it is bleached with
streptomyocin, the extracts no longer contain these enzymes. On the
other hand, the naturally colorless euglenoid Astasia possesses carboxydismutase activity, but no TPN-specific triose phosphate dehydrogenase
activity.
With regard to the blue-green algae, at least in two types, Anacystis
and Nostoc, the TPN-dependent glyceraldehyde-3-phosphate dehydrogenase was found in cell-free extracts (83). Furthermore, carboxydismutase was detected in cell-free extracts of Anacystis nidulans (70).
These indications, considered along with the typical photosynthetic C0 2
fixation pattern of compounds obtained in Nostoc muscorum
(451),
would lead one to believe that the carbon reduction cycle as shown in
Fig. 2 is operative in the blue-green algae. Fewson et al, however, have
shown without any doubt that fructose-1,6-diphosphate aldolase is absent
in four such algae: Anacystis, Anabaena, Nostoc, and Tolypothrix (83).
Thus, a controversial point is raised concerning the validity of the reductive pentose phosphate cycle or the carbon reduction cycle. Peterkofsky
and Racker pointed out that certain enzymes of this cycle are deficient
even in green algae and leaves so that the cycle cannot support the
rate of photosynthesis by the intact cell (88). These and other evidences,
such as the asymmetric labeling of glucose from C
14 0 2 fixation in the
light and the sensitivity toward iodoacetate inhibition (456), are not
compatible with the operation of the reductive pentose phosphate cycle
as such. Hence, an alternate pathway was proposed that did not involve
the hydrolytic cleavage of the presumed six-carbon intermediate of the
carboxydismutase reaction, and did not involve the recondensation of
two molecules of triose phosphates (456). This whole question has been
discussed in great detail by Wassink in his review article on photosynthesis (457). Before leaving this problem altogether, it seems to this
reviewer that another possibility exists for the condensation of threecarbon compounds to form hexoses via the reversal of the EntnerDoudoroff pathway since it has been recently shown that the 2-keto3-deoxy-6-phosphogluconate aldolase is reversible (458). So far, however, no such enzyme has been detected in algae.
Just as in the case of the autotrophic bacteria, carboxydismutase in
algae is apparently an inducible enzyme. In Table XVI, Chlorella varU
egata as an autotroph is listed as containing reductive pentose phosphate
cycle activity. The heterotrophically grown C. variegata, on the other
hand, possessing no chlorophyll, also has no carboxydismutase activity