4. ENERGY-RICH COMPOUNDS
139
sis of citrulline (164-165a) and pyrimidines (165a), ß-aspartyl phosphate in the conversion of aspartate to homoserine (166), and carbonyl
phosphate in the oxidation of propionate (167) by mammalian tissues.
The participation of glutamyl phosphate in glutamine synthesis is uncertain (170), as is the existence of succinyl phosphate as an intermediate in α-ketoglutarate oxidation (168, 169) or in succinate decarboxylation (163).
(4). Distribution. Acetyl phosphate (and also propionyl and butyryl
phosphate) has been found primarily in microorganisms (177, 183, 186191), although the occurrence of phosphotransacetylase in Phaseolus
radiatus (191a) suggests that acetyl phosphate may be present also in
higher plants. While this compound can be formed by mammalian tissues and yeast by the action of 3-phosphoglyceraldehyde dehydrogenase
(194-197), acetyl phosphate is not accumulated because of the almost
universal presence of acyl phosphatases in these cells (76, 201, 202).
The other carboxyl phosphates mentioned above have been studied
largely with specific mammalian tissues, but their participation in several key metabolic processes would suggest a general distribution in
nature.
b. 1,3-Diphosphoglycerate:
(1). Preparation and identification. 1,3-Diphosphoglycerate, whose
structure is shown in Fig. 7, was originally isolated as the end-product
of the Pi-dependent oxidation of 3-phosphoglyceraldehyde (203, 204)
(cf. Eq. 47).
3-Phosphoglyceraldehyde + DPN+ + P»^ 1,3-Diphosphoglycerate + DPNH + H
+
(47)
If alcohol dehydrogenase and acetaldehyde are added, DPN is regenerated (Reaction 48) and the equilibrium of Reaction 47 is shifted to
the right.
DPNH + H+ + Acetaldehyde ^± DPN+ + Ethanol
(48)
With pure enzyme preparations, this coupled reaction can be used for
the synthesis of 1,3-diphosphoglycerate in good yield (205).
An enzymatic assay for 1,3-diphosphoglycerate is based on reversal
of Reaction 47 and involves a spectrophotometric measurement of
DPNH disappearance (203-205).
(2). Biosynthesis. 1,3-Diphosphoglycerate is synthesized solely by
Reaction 47, as shown above. The substrate for this reaction, 3-phosphoglyceraldehyde, arises normally by glycolysis and the pentose cycle
or, to a lesser extent, by direct cleavage of xylulose-5-phosphate.
3-Phosphoglyceraldehyde dehydrogenase, the enzyme-mediating reaction
(Eq. 47), has been crystallized from yeast (206, 207) and muscle (208,
139
sis of citrulline (164-165a) and pyrimidines (165a), ß-aspartyl phosphate in the conversion of aspartate to homoserine (166), and carbonyl
phosphate in the oxidation of propionate (167) by mammalian tissues.
The participation of glutamyl phosphate in glutamine synthesis is uncertain (170), as is the existence of succinyl phosphate as an intermediate in α-ketoglutarate oxidation (168, 169) or in succinate decarboxylation (163).
(4). Distribution. Acetyl phosphate (and also propionyl and butyryl
phosphate) has been found primarily in microorganisms (177, 183, 186191), although the occurrence of phosphotransacetylase in Phaseolus
radiatus (191a) suggests that acetyl phosphate may be present also in
higher plants. While this compound can be formed by mammalian tissues and yeast by the action of 3-phosphoglyceraldehyde dehydrogenase
(194-197), acetyl phosphate is not accumulated because of the almost
universal presence of acyl phosphatases in these cells (76, 201, 202).
The other carboxyl phosphates mentioned above have been studied
largely with specific mammalian tissues, but their participation in several key metabolic processes would suggest a general distribution in
nature.
b. 1,3-Diphosphoglycerate:
(1). Preparation and identification. 1,3-Diphosphoglycerate, whose
structure is shown in Fig. 7, was originally isolated as the end-product
of the Pi-dependent oxidation of 3-phosphoglyceraldehyde (203, 204)
(cf. Eq. 47).
3-Phosphoglyceraldehyde + DPN+ + P»^ 1,3-Diphosphoglycerate + DPNH + H
+
(47)
If alcohol dehydrogenase and acetaldehyde are added, DPN is regenerated (Reaction 48) and the equilibrium of Reaction 47 is shifted to
the right.
DPNH + H+ + Acetaldehyde ^± DPN+ + Ethanol
(48)
With pure enzyme preparations, this coupled reaction can be used for
the synthesis of 1,3-diphosphoglycerate in good yield (205).
An enzymatic assay for 1,3-diphosphoglycerate is based on reversal
of Reaction 47 and involves a spectrophotometric measurement of
DPNH disappearance (203-205).
(2). Biosynthesis. 1,3-Diphosphoglycerate is synthesized solely by
Reaction 47, as shown above. The substrate for this reaction, 3-phosphoglyceraldehyde, arises normally by glycolysis and the pentose cycle
or, to a lesser extent, by direct cleavage of xylulose-5-phosphate.
3-Phosphoglyceraldehyde dehydrogenase, the enzyme-mediating reaction
(Eq. 47), has been crystallized from yeast (206, 207) and muscle (208,