72
Ν. G. PON
ence of the cycle enzymes, (b) the presence of some intermediates of
the cycle, or (c) by the utilization and conversion of pentose phosphate
cycle intermediates to other intermediates.
Examples of the first type (a) are as follows: Spinach leaves contain
phosphoribose isomerase (459), transketolase (466), and phosphoketopentose epimerase (467). Pea leaves contain phosphoribose isomerase
(459) along with 6-PG DH and transketolase (468). Phosphoribose
isomerase derived from alfalfa is one of the best-characterized enzymes
with respect to its biochemical and physical chemical properties (459).
Both G-6-P DH and 6-PG DH have been detected in leaves of tobacco,
Nicotiana tabacum, by use of triphenyl tetrazolium (469). The activity
of the former enzyme has also been observed in extracts of corn leaves
(470).
(b) The presence of intermediates might also be an indication that
the pentose pathway is in operation. Three such examples come to mind:
Fig leaves (460) and Sedum speotabile, one of the Crassulaceae (succulent plant) (471), both possess sedoheptulose. Alfalfa, when grown in
nutrient cultures and fed D-ribose, yields a very marked increase in
sedoheptulose
(472).
(c) The utilization and conversion of pentose phosphate cycle intermediates are of two types: to phosphorylated intermediates or to C0 2 .
Both types of reactions have been investigated in the tobacco leaf.
Ribose-5-phosphate was transformed to ribulose-5-phosphate, sedoheptulose-7-phosphate, fructose-6-phosphate, and glucose-6-phosphate by extracts of tobacco leaves (473). Leaf disks of tobacco administered variously labeled glucose yielded radioactive carbon dioxide, starch, sugars,
and organic acids (474). These researchers estimated that the amount
of added glucose metabolized via the EMP route and via the pentose
phosphate cycle is in the ratio of 3:2. Gibbs and Horecker, in a classic
experiment, reported the presence of an active transketolase system in
pea (Pimm sativum) leaves which can partly account for the correct
intramolecular distribution of label in the hexose molecule when the
leaves are fed specifically labeled ribose-5-phosphate (475). The example
of Tolbert and Zill has already been mentioned in Section V, D; however,
examples were not given. Radioactive sedoheptulose was rapidly converted to C
14
-labeled sugar phosphate, under illumination and in the
presence of nitrogen, by sugar beet leaves, Wintex barley leaves, and
tobacco leaves (67). Finally, Ross et al. have shown that the leaves of
two herbaceous plants, Chenopodium murale L. and Polygonum Orientale
L., are able to oxidize glucose-l-C
14 to C
14 0 2 preferentially over that of
glucose-6-C
14
(476).
As compared to the data on pentose phosphate cycle metabolism by
Ν. G. PON
ence of the cycle enzymes, (b) the presence of some intermediates of
the cycle, or (c) by the utilization and conversion of pentose phosphate
cycle intermediates to other intermediates.
Examples of the first type (a) are as follows: Spinach leaves contain
phosphoribose isomerase (459), transketolase (466), and phosphoketopentose epimerase (467). Pea leaves contain phosphoribose isomerase
(459) along with 6-PG DH and transketolase (468). Phosphoribose
isomerase derived from alfalfa is one of the best-characterized enzymes
with respect to its biochemical and physical chemical properties (459).
Both G-6-P DH and 6-PG DH have been detected in leaves of tobacco,
Nicotiana tabacum, by use of triphenyl tetrazolium (469). The activity
of the former enzyme has also been observed in extracts of corn leaves
(470).
(b) The presence of intermediates might also be an indication that
the pentose pathway is in operation. Three such examples come to mind:
Fig leaves (460) and Sedum speotabile, one of the Crassulaceae (succulent plant) (471), both possess sedoheptulose. Alfalfa, when grown in
nutrient cultures and fed D-ribose, yields a very marked increase in
sedoheptulose
(472).
(c) The utilization and conversion of pentose phosphate cycle intermediates are of two types: to phosphorylated intermediates or to C0 2 .
Both types of reactions have been investigated in the tobacco leaf.
Ribose-5-phosphate was transformed to ribulose-5-phosphate, sedoheptulose-7-phosphate, fructose-6-phosphate, and glucose-6-phosphate by extracts of tobacco leaves (473). Leaf disks of tobacco administered variously labeled glucose yielded radioactive carbon dioxide, starch, sugars,
and organic acids (474). These researchers estimated that the amount
of added glucose metabolized via the EMP route and via the pentose
phosphate cycle is in the ratio of 3:2. Gibbs and Horecker, in a classic
experiment, reported the presence of an active transketolase system in
pea (Pimm sativum) leaves which can partly account for the correct
intramolecular distribution of label in the hexose molecule when the
leaves are fed specifically labeled ribose-5-phosphate (475). The example
of Tolbert and Zill has already been mentioned in Section V, D; however,
examples were not given. Radioactive sedoheptulose was rapidly converted to C
14
-labeled sugar phosphate, under illumination and in the
presence of nitrogen, by sugar beet leaves, Wintex barley leaves, and
tobacco leaves (67). Finally, Ross et al. have shown that the leaves of
two herbaceous plants, Chenopodium murale L. and Polygonum Orientale
L., are able to oxidize glucose-l-C
14 to C
14 0 2 preferentially over that of
glucose-6-C
14
(476).
As compared to the data on pentose phosphate cycle metabolism by