1. PENTOSE PHOSPHATE CYCLE
73
leaves, those of stems and stalks are relatively sparse. In the stalks of
Sedum spectabile are present sedoheptulose (471). Bean (Phaseolus
vulgaris var. Black Valentine) stem tissues contain enzymes that will
yield a C-6:C-l ratio of the respiratory C0 2 equal to approximately 0.5
from the respectively labeled glucose (477). Indirect evidence from
phosphorylated substrate utilization and conversion in the mitochondrial
preparation from the apical parts of etiolated young pea plants strongly
implicate the presence of G-6-P DH, 6-PG DH, phosphohexose isomerase,
pentose phosphate isomerase, phosphoketopentose epimerase, and transketolase (478). This is a surprising result in view of the fact that most
pentose phosphate cycle enzymes are localized in the soluble (probably
cytoplasmic) fraction of mammalian tissue cells.
A freshly cut carrot has the potential to oxidize glucose via the shunt
mechanism, especially when stimulated by the addition of methylene
blue (479). In fact, if the carrots are not freshly cut, the respiration
rises and the oxidation of carbon atom 1 of glucose is accelerated more
than that of carbon atom 6 (480). Washing of the slices induces a fully
developed respiration rate and the respiratory CO s ratio of C-l:C-6
reaches 4 to 5. The same observations were made with potato slices
(480, 481). Mitochondria isolated from potato tissues have been shown
to contain enzymes that will oxidize fructose-6-phosphate in the presence
of TPN
+ and will transform ribose-5-phosphate to ribulose-5-phosphate
and sedoheptulose-7-phosphate (478).
The root tips of corn seedlings, although having a shunt mechanism
in operation, is severely limited by the supply of TPN
+ (482). The shunt
activity can be stimulated by adding nitrite. Enzymes such as G-6-P DH,
6-PG DH, enzymes that utilize ribose-5-phosphate, and transforming enzymes can be found in these root tips (483). Extracts of pea seedling
root tips will metabolize variously labeled ribose-5-phosphate to form
radioactive hexose monophosphate with a distribution of label which is
accountable only by invoking the combined actions of transketolase and
transaldolase (475). In fact, this extract exhibits G-6-P DH and 6-PG
DH activities along with many EMP scheme enzymes (484). Humphreys
and Dugger, showed that root tips from a number of seedlings (pea,
Pisum sativum L.; corn, Zea mays L.; and oat, Avena sativum L.) gave
C-6:C-l ratios of respiratory C0 2 generally less than unity (485).
Other seedlings with indications of the occurrence of the hexose
monophosphate pathway are those of pine (486), etiolated Sorghum
vulgare (487), and mung bean, Phaseolus radiatus (488). Wheat germ
extracts can catalyze the oxidative degradation of glucose-6-phosphate to
C0 2 which is TPN-dependent (489).
The effect of aging on the activity of the pentose phosphate cycle
73
leaves, those of stems and stalks are relatively sparse. In the stalks of
Sedum spectabile are present sedoheptulose (471). Bean (Phaseolus
vulgaris var. Black Valentine) stem tissues contain enzymes that will
yield a C-6:C-l ratio of the respiratory C0 2 equal to approximately 0.5
from the respectively labeled glucose (477). Indirect evidence from
phosphorylated substrate utilization and conversion in the mitochondrial
preparation from the apical parts of etiolated young pea plants strongly
implicate the presence of G-6-P DH, 6-PG DH, phosphohexose isomerase,
pentose phosphate isomerase, phosphoketopentose epimerase, and transketolase (478). This is a surprising result in view of the fact that most
pentose phosphate cycle enzymes are localized in the soluble (probably
cytoplasmic) fraction of mammalian tissue cells.
A freshly cut carrot has the potential to oxidize glucose via the shunt
mechanism, especially when stimulated by the addition of methylene
blue (479). In fact, if the carrots are not freshly cut, the respiration
rises and the oxidation of carbon atom 1 of glucose is accelerated more
than that of carbon atom 6 (480). Washing of the slices induces a fully
developed respiration rate and the respiratory CO s ratio of C-l:C-6
reaches 4 to 5. The same observations were made with potato slices
(480, 481). Mitochondria isolated from potato tissues have been shown
to contain enzymes that will oxidize fructose-6-phosphate in the presence
of TPN
+ and will transform ribose-5-phosphate to ribulose-5-phosphate
and sedoheptulose-7-phosphate (478).
The root tips of corn seedlings, although having a shunt mechanism
in operation, is severely limited by the supply of TPN
+ (482). The shunt
activity can be stimulated by adding nitrite. Enzymes such as G-6-P DH,
6-PG DH, enzymes that utilize ribose-5-phosphate, and transforming enzymes can be found in these root tips (483). Extracts of pea seedling
root tips will metabolize variously labeled ribose-5-phosphate to form
radioactive hexose monophosphate with a distribution of label which is
accountable only by invoking the combined actions of transketolase and
transaldolase (475). In fact, this extract exhibits G-6-P DH and 6-PG
DH activities along with many EMP scheme enzymes (484). Humphreys
and Dugger, showed that root tips from a number of seedlings (pea,
Pisum sativum L.; corn, Zea mays L.; and oat, Avena sativum L.) gave
C-6:C-l ratios of respiratory C0 2 generally less than unity (485).
Other seedlings with indications of the occurrence of the hexose
monophosphate pathway are those of pine (486), etiolated Sorghum
vulgare (487), and mung bean, Phaseolus radiatus (488). Wheat germ
extracts can catalyze the oxidative degradation of glucose-6-phosphate to
C0 2 which is TPN-dependent (489).
The effect of aging on the activity of the pentose phosphate cycle