12
Ν. G. PON
recent example of the latter case is an experiment with homogenates of
lobster hepatopancreas to which addition of 6-phosphogluconate and
ribose-5-phosphate stimulated oxygen uptake (66).
One serious drawback of this approach is that all the intermediates of
the pentose phosphate cycle are phosphorylated compounds and therefore are impermeable to the intact cells. This property of these substances requires working with either cell-free homogenates or tissue
slices or minces, which makes the problem fall within the realm of
enzymology, the pitfalls of which are described below. Alternatively,
unphosphorylated precursors of these cycle intermediates may be supplied to the intact tissue; however, assumptions have to be made that the
cell utilizes the supplied substance subsequently in the form of its
phosphate ester.
D. TRANSFORMATION OF PENTOSE PHOSPHATE CYCLE INTERMEDIATES
Here again we are faced with the same problem as above, namely,
that of getting these intermediates into the cell, because of the impermeable nature of these compounds. The basic idea behind this whole
approach is to study the formation of compounds derived from an intermediate of the pentose phosphate cycle. If these compounds are also
pentose phosphate intermediates, then it is probable that the cycle is in
operation under the conditions of the experiment. One early example of
this type of approach was the study of the metabolism of sedoheptuloseC
14
in beet, barley, and tobacco leaves (67). The labeled compounds
formed varied considerably depending on the conditions; i.e., whether
in the light or in the dark or whether under nitrogen or under air. Some
of the radioactive phosphorylated esters obtained in light under nitrogen
were those of ribulose, ribose, and fructose. These compounds are characteristic of the operation of both the oxidative pentose phosphate cycle
and the photosynthetic carbon reduction cycle. Later, as more refined
techniques became available, kinetic analyses of shorter and shorter time
of incorporation of C
14 from labeled substrates were possible. The classic
example of the use of this method is the study of C0 2 fixation during
photosynthesis using C
14 0 2 as substrate; it has been described elsewhere
in this review (cf. brief history section on the reductive pentose phosphate cycle, Section IV, B). The same methods have been applied on
nonphotosynthetic organisms. In one case, using glucose-C
14
, Moses
demonstrated the presence of the oxidative pentose phosphate cycle in
Zygorhynchus moelleri (68), and in the other case, using C
14 0 2 , Aubert
et al. showed that the reductive pentose phosphate cycle was operating
in Thiobacillus denitrificans (69).
Ν. G. PON
recent example of the latter case is an experiment with homogenates of
lobster hepatopancreas to which addition of 6-phosphogluconate and
ribose-5-phosphate stimulated oxygen uptake (66).
One serious drawback of this approach is that all the intermediates of
the pentose phosphate cycle are phosphorylated compounds and therefore are impermeable to the intact cells. This property of these substances requires working with either cell-free homogenates or tissue
slices or minces, which makes the problem fall within the realm of
enzymology, the pitfalls of which are described below. Alternatively,
unphosphorylated precursors of these cycle intermediates may be supplied to the intact tissue; however, assumptions have to be made that the
cell utilizes the supplied substance subsequently in the form of its
phosphate ester.
D. TRANSFORMATION OF PENTOSE PHOSPHATE CYCLE INTERMEDIATES
Here again we are faced with the same problem as above, namely,
that of getting these intermediates into the cell, because of the impermeable nature of these compounds. The basic idea behind this whole
approach is to study the formation of compounds derived from an intermediate of the pentose phosphate cycle. If these compounds are also
pentose phosphate intermediates, then it is probable that the cycle is in
operation under the conditions of the experiment. One early example of
this type of approach was the study of the metabolism of sedoheptuloseC
14
in beet, barley, and tobacco leaves (67). The labeled compounds
formed varied considerably depending on the conditions; i.e., whether
in the light or in the dark or whether under nitrogen or under air. Some
of the radioactive phosphorylated esters obtained in light under nitrogen
were those of ribulose, ribose, and fructose. These compounds are characteristic of the operation of both the oxidative pentose phosphate cycle
and the photosynthetic carbon reduction cycle. Later, as more refined
techniques became available, kinetic analyses of shorter and shorter time
of incorporation of C
14 from labeled substrates were possible. The classic
example of the use of this method is the study of C0 2 fixation during
photosynthesis using C
14 0 2 as substrate; it has been described elsewhere
in this review (cf. brief history section on the reductive pentose phosphate cycle, Section IV, B). The same methods have been applied on
nonphotosynthetic organisms. In one case, using glucose-C
14
, Moses
demonstrated the presence of the oxidative pentose phosphate cycle in
Zygorhynchus moelleri (68), and in the other case, using C
14 0 2 , Aubert
et al. showed that the reductive pentose phosphate cycle was operating
in Thiobacillus denitrificans (69).