10
Ν. G. PON
1956, that there are at least six different ways of indicating the presence
of the oxidative pentose phosphate cycle. These are: (a) experiments
with inhibitors; (b) the presence of pentose phosphate cycle intermediates in the tissues; (c) the use of these intermediates as substrates for
respiration; (d) the transformation of these intermediates to others; (e)
the presence of pentose phosphate pathway enzymes; and (/) experiments with specifically labeled substrates for respiration. These criteria
apply as well for the reductive pentose phosphate cycle.
A. INHIBITORS AS INDICATORS OF ALTERNATE PATHWAYS OF METABOLISM
The classic case of the use of such substances is that of iodoacetic
acid or iodoacetamide. An early example is the work of Lundsgaard in
which he showed that respiration continued uninterrupted under conditions where the iodoacetate concentration was sufficiently high to poison
the process of fermentation (59). He further demonstrated that oxidation
of hexose occurs in yeast which is inhibited by this same chemical.
Essentially the same technique was applied to the study of the aerobic
and anaerobic formation of acids using iodoacetate and cyanide as inhibitors in the fertilized egg of sea urchins (60). Iodoacetic acid also
affects carbohydrate metabolism in a variety of isolated mammalian
tissues such as cat brain cortex, smooth muscle and heart muscle tissues,
and also in dog skeletal muscle and heart muscle tissues (61). Thus
anaerobic glycolysis is inhibited to the extent of 90-100% as measured
by lactate production or C0 2 evolution (acid formation) whereas respiration continues unaffected. To this type of experiment, however,
Racker raised the following objections (4): In muscle tissues, the activity
of alternate pathways of glucose oxidation is very low, but the effect of
iodoacetate is pronounced, although glucose oxidation is not affected.
Furthermore, in a system where the formation of ATP is limiting, a
slight inhibition of the triose phosphate dehydrogenase system markedly
affects the ATP level. This condition would lead to a large decrease in
hexose phosphorylation and thus to a large decrease in glucose uptake.
Finally, it should be mentioned that this approach of using inhibitors for
the determination of alternate pathways of metabolism does not pinpoint
exactly what type of pathway is involved since, most certainly, many
organisms contain pathways that are as yet unknown.
B. PRESENCE OF PENTOSE PHOSPHATE CYCLE INTERMEDIATES IN TISSUES
It is not enough to show that a particular intermediate is present in
the tissue or that its concentration varies under changes of conditions
such as from aerobic to anaerobic conditions. Moreover, the formation of
this intermediate from precursors must be followed with studies of the
Ν. G. PON
1956, that there are at least six different ways of indicating the presence
of the oxidative pentose phosphate cycle. These are: (a) experiments
with inhibitors; (b) the presence of pentose phosphate cycle intermediates in the tissues; (c) the use of these intermediates as substrates for
respiration; (d) the transformation of these intermediates to others; (e)
the presence of pentose phosphate pathway enzymes; and (/) experiments with specifically labeled substrates for respiration. These criteria
apply as well for the reductive pentose phosphate cycle.
A. INHIBITORS AS INDICATORS OF ALTERNATE PATHWAYS OF METABOLISM
The classic case of the use of such substances is that of iodoacetic
acid or iodoacetamide. An early example is the work of Lundsgaard in
which he showed that respiration continued uninterrupted under conditions where the iodoacetate concentration was sufficiently high to poison
the process of fermentation (59). He further demonstrated that oxidation
of hexose occurs in yeast which is inhibited by this same chemical.
Essentially the same technique was applied to the study of the aerobic
and anaerobic formation of acids using iodoacetate and cyanide as inhibitors in the fertilized egg of sea urchins (60). Iodoacetic acid also
affects carbohydrate metabolism in a variety of isolated mammalian
tissues such as cat brain cortex, smooth muscle and heart muscle tissues,
and also in dog skeletal muscle and heart muscle tissues (61). Thus
anaerobic glycolysis is inhibited to the extent of 90-100% as measured
by lactate production or C0 2 evolution (acid formation) whereas respiration continues unaffected. To this type of experiment, however,
Racker raised the following objections (4): In muscle tissues, the activity
of alternate pathways of glucose oxidation is very low, but the effect of
iodoacetate is pronounced, although glucose oxidation is not affected.
Furthermore, in a system where the formation of ATP is limiting, a
slight inhibition of the triose phosphate dehydrogenase system markedly
affects the ATP level. This condition would lead to a large decrease in
hexose phosphorylation and thus to a large decrease in glucose uptake.
Finally, it should be mentioned that this approach of using inhibitors for
the determination of alternate pathways of metabolism does not pinpoint
exactly what type of pathway is involved since, most certainly, many
organisms contain pathways that are as yet unknown.
B. PRESENCE OF PENTOSE PHOSPHATE CYCLE INTERMEDIATES IN TISSUES
It is not enough to show that a particular intermediate is present in
the tissue or that its concentration varies under changes of conditions
such as from aerobic to anaerobic conditions. Moreover, the formation of
this intermediate from precursors must be followed with studies of the