1. PENTOSE PHOSPHATE CYCLE
71
(425). Perhaps there is a parallel relationship between the formation of
the chlorophyll and the enzyme in a manner similar to the suggestions
made by Lascelles (438).
XVI. Higher Plants
A. THE OXIDATIVE PENTOSE PHOSPHATE CYCLE
The catabolism of glucose by higher plants much resembles that of
the mammals. Tissues of higher plants have served as source material
for studies in this regard for innumerable experiments. Generally, these
tissues have the capacity to degrade carbohydrates via the oxidative
pathway no matter what the source: the fruit, the leaves, the stems or
stalks, the roots or tubers, or the seeds or seedlings. Some examples of
each of these categories should illustrate the point.
One of the enzymes of the nonoxidative portion of the pentose phosphate pathway, phosphoribose isomerase, has been detected in the
flavedo of the orange and in the avocado fruit (459). The Kadota fig
fruit contains D-aZfro-heptulose (460). In studying the pathway of synthesis of L-ascorbate in the fruit of the strawberry, Fragaria, Loewus
concluded that L-ascorbate is formed via two pathways, one involving
the inversion of a six-carbon sugar and the other not inverting this
sugar (461). The latter, he postulated as going through the direct oxidative pathway via 6-phosphogluconate and eventually to L-ascorbate.
Wang and his co-workers have examined the role of the hexose monophosphate pathway in the catabolism of glucose in a number of fruits.
Using the radiorespirometric method, they estimated that over 25% of
the glucose is degraded via this alternate pathway in the tomato (462)
and between 30 and 35% in the fruit of four varieties of the pepper
Capsicum frutescens Longum (463). Other fruits yielding relatively large
C-l:C-6 ratios of respiratory C0 2 from the respectively labeled glucose
are the cucumber, the lime, and the orange (464). In the pre-ripe
banana, the pentose phosphate cycle seems to be quite active as suggested by the lack of inhibition by fluoride of respiration, by the ability
of the fruit to utilize both glucose-6-phosphate and ribose-5-phosphate,
and by the inability of the fruit to metabolize fructose-l,6-diphosphate
(465). As the climacteric is approached, however, the whole metabolic
pattern shifts toward the EMP scheme, which manifests itself by the
appearance of an active aldolase, carboxylase, the metabolism of hexose
diphosphate, and an increased sensitivity toward fluoride inhibition of
the respiration in the pulp.
The pentose phosphate cycle is evident in the leaves by (a) the pres-
71
(425). Perhaps there is a parallel relationship between the formation of
the chlorophyll and the enzyme in a manner similar to the suggestions
made by Lascelles (438).
XVI. Higher Plants
A. THE OXIDATIVE PENTOSE PHOSPHATE CYCLE
The catabolism of glucose by higher plants much resembles that of
the mammals. Tissues of higher plants have served as source material
for studies in this regard for innumerable experiments. Generally, these
tissues have the capacity to degrade carbohydrates via the oxidative
pathway no matter what the source: the fruit, the leaves, the stems or
stalks, the roots or tubers, or the seeds or seedlings. Some examples of
each of these categories should illustrate the point.
One of the enzymes of the nonoxidative portion of the pentose phosphate pathway, phosphoribose isomerase, has been detected in the
flavedo of the orange and in the avocado fruit (459). The Kadota fig
fruit contains D-aZfro-heptulose (460). In studying the pathway of synthesis of L-ascorbate in the fruit of the strawberry, Fragaria, Loewus
concluded that L-ascorbate is formed via two pathways, one involving
the inversion of a six-carbon sugar and the other not inverting this
sugar (461). The latter, he postulated as going through the direct oxidative pathway via 6-phosphogluconate and eventually to L-ascorbate.
Wang and his co-workers have examined the role of the hexose monophosphate pathway in the catabolism of glucose in a number of fruits.
Using the radiorespirometric method, they estimated that over 25% of
the glucose is degraded via this alternate pathway in the tomato (462)
and between 30 and 35% in the fruit of four varieties of the pepper
Capsicum frutescens Longum (463). Other fruits yielding relatively large
C-l:C-6 ratios of respiratory C0 2 from the respectively labeled glucose
are the cucumber, the lime, and the orange (464). In the pre-ripe
banana, the pentose phosphate cycle seems to be quite active as suggested by the lack of inhibition by fluoride of respiration, by the ability
of the fruit to utilize both glucose-6-phosphate and ribose-5-phosphate,
and by the inability of the fruit to metabolize fructose-l,6-diphosphate
(465). As the climacteric is approached, however, the whole metabolic
pattern shifts toward the EMP scheme, which manifests itself by the
appearance of an active aldolase, carboxylase, the metabolism of hexose
diphosphate, and an increased sensitivity toward fluoride inhibition of
the respiration in the pulp.
The pentose phosphate cycle is evident in the leaves by (a) the pres-