64
Ν. G. PON
phosphate cycle except possibly phosphoketopentose epimerase have
been found in this organism. One of the enzymes, phosphopentose
isomerase, has been purified and characterized with respect to pH
optimum, thermolability, sensitivity to inhibitors, equilibrium constant,
etc., and was found to be clearly different from the corresponding alfalfa
enzyme (413).
XIII. Protozoa
The organism Entamoeba histolytica is best known for its pathogenicity. This ameba is responsible for the disease amebic dysentery.
When tested for G-6-P DH and for 6-PG DH, the former was found to
be present, whereas the latter was found to be absent (414). The fact
that ribose-5-phosphate is a growth requirement for this organism is
indicative of the lack of the shunt mechanism. Actually, the end product
of G-6-P metabolism is pyruvate and glyceraldehyde-3-phosphate (415).
This result, along with findings on certain cofactor requirements, suggests that the Entner-Doudoroff pathway is in operation in Entamoeba.
In contrast to these findings, the Laredo strain of this ameba from human
host contains no G-6-P DH and no 6-PG DH activities even in the presence of added TPN
+
(416). Neither is ribose-5-phosphate metabolized
by cell-free extracts of this organism, but 6-phosphogluconate is cleaved
to pyruvate and triose phosphate. Experiments with specifically labeled
glucose confirmed the notion that the pentose phosphate cycle is absent
in this organism. Also all the EMP pathway enzymes have been demonstrated in the cell-free extracts. The fact that glucose-l-C
14
yields
acetate-2-C
14
means that the predominant pathway is probably the
EMP pathway. Associated with the growth of this ameba in a culture
system is the organism Bacteriodes symbiosus. These bacteria also metabolize via the EMP pathway and produce 1 mole of hydrogen per mole
of glucose utilized (as does the ameba). However, instead of using the
Entner-Doudoroff pathway as an alternate route, these bacteria use the
pentose phosphate cycle (417). Indeed, such bacteria, grown in cysteine
(but not grown in thiomalate) were shown to possess relatively high
levels of both G-6-P DH and 6-PG DH (418), although the cysteinegrown bacteria do not utilize ribose-5-phosphate, whereas the thiomalategrown bacteria readily form ketohexose, ketoheptose, and triose from
ribose-5-phosphate. Moreover, the latter bacteria catabolize 6-phosphogluconate to pentose and ketohexose. Thus it appears that the pentose
phosphate pathway may be present in the thiomalate-grown Bacteriodes
symbiosus. Efforts to demonstrate the existence of the Entner-Doudoroff
cleavage enzyme, 2-keto-3-deoxy-6-phosphogluconate aldolase, in cellfree extracts of these bacteria failed. The data from experiments with
Ν. G. PON
phosphate cycle except possibly phosphoketopentose epimerase have
been found in this organism. One of the enzymes, phosphopentose
isomerase, has been purified and characterized with respect to pH
optimum, thermolability, sensitivity to inhibitors, equilibrium constant,
etc., and was found to be clearly different from the corresponding alfalfa
enzyme (413).
XIII. Protozoa
The organism Entamoeba histolytica is best known for its pathogenicity. This ameba is responsible for the disease amebic dysentery.
When tested for G-6-P DH and for 6-PG DH, the former was found to
be present, whereas the latter was found to be absent (414). The fact
that ribose-5-phosphate is a growth requirement for this organism is
indicative of the lack of the shunt mechanism. Actually, the end product
of G-6-P metabolism is pyruvate and glyceraldehyde-3-phosphate (415).
This result, along with findings on certain cofactor requirements, suggests that the Entner-Doudoroff pathway is in operation in Entamoeba.
In contrast to these findings, the Laredo strain of this ameba from human
host contains no G-6-P DH and no 6-PG DH activities even in the presence of added TPN
+
(416). Neither is ribose-5-phosphate metabolized
by cell-free extracts of this organism, but 6-phosphogluconate is cleaved
to pyruvate and triose phosphate. Experiments with specifically labeled
glucose confirmed the notion that the pentose phosphate cycle is absent
in this organism. Also all the EMP pathway enzymes have been demonstrated in the cell-free extracts. The fact that glucose-l-C
14
yields
acetate-2-C
14
means that the predominant pathway is probably the
EMP pathway. Associated with the growth of this ameba in a culture
system is the organism Bacteriodes symbiosus. These bacteria also metabolize via the EMP pathway and produce 1 mole of hydrogen per mole
of glucose utilized (as does the ameba). However, instead of using the
Entner-Doudoroff pathway as an alternate route, these bacteria use the
pentose phosphate cycle (417). Indeed, such bacteria, grown in cysteine
(but not grown in thiomalate) were shown to possess relatively high
levels of both G-6-P DH and 6-PG DH (418), although the cysteinegrown bacteria do not utilize ribose-5-phosphate, whereas the thiomalategrown bacteria readily form ketohexose, ketoheptose, and triose from
ribose-5-phosphate. Moreover, the latter bacteria catabolize 6-phosphogluconate to pentose and ketohexose. Thus it appears that the pentose
phosphate pathway may be present in the thiomalate-grown Bacteriodes
symbiosus. Efforts to demonstrate the existence of the Entner-Doudoroff
cleavage enzyme, 2-keto-3-deoxy-6-phosphogluconate aldolase, in cellfree extracts of these bacteria failed. The data from experiments with