9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
433
reduced amount because of retention of the bottom 3 carbons of every
hexose molecule that enters the cycle.
When the individual glucose atoms are identified by specific labeling
with C
14 , the foregoing statements can be subjected to experimental
verification (15). The data that can be obtained from appropriate experiments can be employed to investigate the comparative aspects of
oxidative pathway participation, as will be seen in Section V, Comparative Studies.
C. ENZYMES OF THE PENTOSE CYCLE
The enzymes of the pentose cycle have been investigated in detail,
both per se and with regard to the formation of products. They are
listed below.
1. Hexokinase
This enzyme has been studied in many laboratories. Because it has
usually been assumed to be the primary step in glycolysis, hexokinase
activity has in the past been taken to indicate the presence of the latter
pathway. However, in view of the reactions outlined in Fig. 2, it is clear
that the presence in an organism of hexokinase may be just as indicative of the formation of glucose-6-phosphate for pentose cycle oxidations as for glycolysis.
A review of hexokinase and of other 1- and 6-kinases of hexoses has
been presented (2).
2. Glucose-6-phosphate
Dehydrogenäse
This enzyme is one of the most ubiquitous that has been found in
nature. It has been described as occurring, for example, in animal tissues (16-34), worms (35, 36), marine forms (37, 38), insects (39-44),
higher plants (45-51), bacteria (52-70), and fungi, including yeast
(71-86). Of these, however, until recently the enzyme has been only
partially purified (especially from mammalian sources) and the major
source of relatively pure G-6-P dehydrogenase has been from yeast.
The yeast enzyme has been purified 600-900-fold by means of fractionation with ammonium sulfate, adsorption on calcium phosphate gel,
and chromatography on starch-Celite columns.*
Some pertinent points about the yeast enzyme are given in Table I,
and a comparison is made with the enzyme from some other sources.
It may be seen that G-6-P dehydrogenase is present in different types
of tissue in quite different amounts; to produce the purified yeast
* Very recently Julian, Wolfe, and Reithel (23) isolated G-6-P dehydrogenase
in crystalline form from mammary gland (see Table I).
433
reduced amount because of retention of the bottom 3 carbons of every
hexose molecule that enters the cycle.
When the individual glucose atoms are identified by specific labeling
with C
14 , the foregoing statements can be subjected to experimental
verification (15). The data that can be obtained from appropriate experiments can be employed to investigate the comparative aspects of
oxidative pathway participation, as will be seen in Section V, Comparative Studies.
C. ENZYMES OF THE PENTOSE CYCLE
The enzymes of the pentose cycle have been investigated in detail,
both per se and with regard to the formation of products. They are
listed below.
1. Hexokinase
This enzyme has been studied in many laboratories. Because it has
usually been assumed to be the primary step in glycolysis, hexokinase
activity has in the past been taken to indicate the presence of the latter
pathway. However, in view of the reactions outlined in Fig. 2, it is clear
that the presence in an organism of hexokinase may be just as indicative of the formation of glucose-6-phosphate for pentose cycle oxidations as for glycolysis.
A review of hexokinase and of other 1- and 6-kinases of hexoses has
been presented (2).
2. Glucose-6-phosphate
Dehydrogenäse
This enzyme is one of the most ubiquitous that has been found in
nature. It has been described as occurring, for example, in animal tissues (16-34), worms (35, 36), marine forms (37, 38), insects (39-44),
higher plants (45-51), bacteria (52-70), and fungi, including yeast
(71-86). Of these, however, until recently the enzyme has been only
partially purified (especially from mammalian sources) and the major
source of relatively pure G-6-P dehydrogenase has been from yeast.
The yeast enzyme has been purified 600-900-fold by means of fractionation with ammonium sulfate, adsorption on calcium phosphate gel,
and chromatography on starch-Celite columns.*
Some pertinent points about the yeast enzyme are given in Table I,
and a comparison is made with the enzyme from some other sources.
It may be seen that G-6-P dehydrogenase is present in different types
of tissue in quite different amounts; to produce the purified yeast
* Very recently Julian, Wolfe, and Reithel (23) isolated G-6-P dehydrogenase
in crystalline form from mammary gland (see Table I).