14
Ν. G. PON
and carboxydismutase. The presence of one or two of these enzymes in
extracts of the tissue, however, does not necessarily mean that the whole
cycle is operating. For example, glucose-6-phosphate dehydrogenase can
be present without pentose phosphate formation, i.e., via the EntnerDoudoroff pathway. Similarly, both transketolase and transaldolase may
be present without having the oxidative portion of the pentose phosphate
cycle, as indicated previously in ribose synthesis by Alcaligenes faecalis
(64). The presence of carboxydismutase in several blue-green algae,
which contain no aldolase (83), strongly suggests that the formation of
hexose from photosynthesis in these organisms may well have to be via
a pathway other than the reductive pentose phosphate cycle.
When no enzyme of a particular pathway is found in extracts of the
tissue, can we say that this enzyme is really absent? Probably not; perhaps the proper conditions either for the preservation of its activity or for
its activation have not been found. Marks et al. (84) found that glucose6-phosphate dehydrogenase from normal individuals is activated to a
greater degree under certain conditions than the corresponding enzyme
from individuals who have a genetic deficiency of this enzyme. However, with proper precautions and cautious interpretation of the data,
the lack of an enzyme in a given preparation might actually be significant. One other fact might be considered: The lack of a given enzyme
does not eliminate the possibility that another enzyme having slightly
different specificities is present.
If one or two indicator enzymes can serve for the prediction of a
route of glucose metabolism, then it would seem reasonable that a map
or profile of all the enzymes of the pathway would be even more helpful.
Studies of this kind were performed on several photosynthetic organisms
(70, 85), the two-spotted spider mite (86), photosynthetic purple bacteria (87), and several photosynthetic green tissues (88). In spite of the
presence of all the enzymes of a given pathway, there is still some doubt
as to whether all these enzymes are located in a common pool. It may
be argued, however, that the pentose phosphate cycle enzymes are
generally found in the soluble fraction of the cell, but recent work has
shown that the particulate fraction of male guinea pig brain contains at
least 10% of the glucose-6-phosphate and 6-phosphogluconate dehydrogenases (89). The unusual feature of this finding arose because these
enzymatic activities manifested themselves only when the particulate
fractions were first treated with a detergent and then separated from the
solubilized enzymes. Care should be exercised to ensure that the enzymes
in question are not absorbed on the particles during their preparation as
in the case of the absorption of lactate dehydrogenase by microsomes
Ν. G. PON
and carboxydismutase. The presence of one or two of these enzymes in
extracts of the tissue, however, does not necessarily mean that the whole
cycle is operating. For example, glucose-6-phosphate dehydrogenase can
be present without pentose phosphate formation, i.e., via the EntnerDoudoroff pathway. Similarly, both transketolase and transaldolase may
be present without having the oxidative portion of the pentose phosphate
cycle, as indicated previously in ribose synthesis by Alcaligenes faecalis
(64). The presence of carboxydismutase in several blue-green algae,
which contain no aldolase (83), strongly suggests that the formation of
hexose from photosynthesis in these organisms may well have to be via
a pathway other than the reductive pentose phosphate cycle.
When no enzyme of a particular pathway is found in extracts of the
tissue, can we say that this enzyme is really absent? Probably not; perhaps the proper conditions either for the preservation of its activity or for
its activation have not been found. Marks et al. (84) found that glucose6-phosphate dehydrogenase from normal individuals is activated to a
greater degree under certain conditions than the corresponding enzyme
from individuals who have a genetic deficiency of this enzyme. However, with proper precautions and cautious interpretation of the data,
the lack of an enzyme in a given preparation might actually be significant. One other fact might be considered: The lack of a given enzyme
does not eliminate the possibility that another enzyme having slightly
different specificities is present.
If one or two indicator enzymes can serve for the prediction of a
route of glucose metabolism, then it would seem reasonable that a map
or profile of all the enzymes of the pathway would be even more helpful.
Studies of this kind were performed on several photosynthetic organisms
(70, 85), the two-spotted spider mite (86), photosynthetic purple bacteria (87), and several photosynthetic green tissues (88). In spite of the
presence of all the enzymes of a given pathway, there is still some doubt
as to whether all these enzymes are located in a common pool. It may
be argued, however, that the pentose phosphate cycle enzymes are
generally found in the soluble fraction of the cell, but recent work has
shown that the particulate fraction of male guinea pig brain contains at
least 10% of the glucose-6-phosphate and 6-phosphogluconate dehydrogenases (89). The unusual feature of this finding arose because these
enzymatic activities manifested themselves only when the particulate
fractions were first treated with a detergent and then separated from the
solubilized enzymes. Care should be exercised to ensure that the enzymes
in question are not absorbed on the particles during their preparation as
in the case of the absorption of lactate dehydrogenase by microsomes