9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
443
The enzyme has been purified 25-fold from P. fluorescens (116). The
products, pyruvate and triose phosphate, may then be utilized either by
glycolysis or the pentose cycle. An important difference between this
scheme and the two others just named lies in the source of the carbon
atoms: pyruvate carbons 1, 2, and 3 correspond to carbons 1, 2, and 3
of glucose in that order, whereas from glycolysis the sequence is reversed. The order of the carbon atoms in triose phosphate is the same
as in glycolysis. This difference permits analysis, by labeled C
14 -glucose,
of the routes of carbohydrate breakdown in an organism (15).
The enzymes of the Entner-Doudoroff pathway have been sought in
a variety of species, but to date they appear to be restricted to a few
pseudomonads (114, 117), Azotobacter (118), Xanthomonas, Agrobacterium, Rhizobium (119), and E. colt (120). Function of this route has
not been demonstrated either in mammals or in higher plants, and it
may be presumed, in view of the close relationships of the metabolic
processes and the ubiquity of both glycolysis and the pentose cycle
enzymes in many species, that 6-PGA dehydrase and/or KDPG may be
absent in most organisms, despite the postulated existence of 2- or 3keto-6-PGA in other species (91-94).
V. Comparative Studies
A. EXAMINATION OF METHODOLOGY
The recognition of alternate pathways for carbohydrate catabolism,
particularly those for the hexoses and the pentoses, inspired students in
biochemistry to estimate the relative participation of concurrent routes
and to elucidate the specific roles played by individual pathways in
cellular biosynthesis and respiration. Much has been accomplished recently in this area owing, at least in part, to the development of better
research methods, such as radioactive tracer techniques and improved
procedures for enzymological studies.
It is desirable first to discuss briefly the methodology involved in the
study of pathway participation in both intact and cell-free biological
systems. Here one finds that most of the reported methods involve two
different approaches: (a) the detection of individual pathways, and (b)
the estimation of pathway participation, analogous to the qualitative
and quantitative analysis of a given problem. In the former, demonstration of the presence of individual enzymes associated with a pathway in
a given biological system often is taken as presumptive evidence of the
operation of the pathway in question. Details in this regard have been
given in the preceding section. However, it should be stressed that the
occurrence of a given pathway represents the sequential operation of a
443
The enzyme has been purified 25-fold from P. fluorescens (116). The
products, pyruvate and triose phosphate, may then be utilized either by
glycolysis or the pentose cycle. An important difference between this
scheme and the two others just named lies in the source of the carbon
atoms: pyruvate carbons 1, 2, and 3 correspond to carbons 1, 2, and 3
of glucose in that order, whereas from glycolysis the sequence is reversed. The order of the carbon atoms in triose phosphate is the same
as in glycolysis. This difference permits analysis, by labeled C
14 -glucose,
of the routes of carbohydrate breakdown in an organism (15).
The enzymes of the Entner-Doudoroff pathway have been sought in
a variety of species, but to date they appear to be restricted to a few
pseudomonads (114, 117), Azotobacter (118), Xanthomonas, Agrobacterium, Rhizobium (119), and E. colt (120). Function of this route has
not been demonstrated either in mammals or in higher plants, and it
may be presumed, in view of the close relationships of the metabolic
processes and the ubiquity of both glycolysis and the pentose cycle
enzymes in many species, that 6-PGA dehydrase and/or KDPG may be
absent in most organisms, despite the postulated existence of 2- or 3keto-6-PGA in other species (91-94).
V. Comparative Studies
A. EXAMINATION OF METHODOLOGY
The recognition of alternate pathways for carbohydrate catabolism,
particularly those for the hexoses and the pentoses, inspired students in
biochemistry to estimate the relative participation of concurrent routes
and to elucidate the specific roles played by individual pathways in
cellular biosynthesis and respiration. Much has been accomplished recently in this area owing, at least in part, to the development of better
research methods, such as radioactive tracer techniques and improved
procedures for enzymological studies.
It is desirable first to discuss briefly the methodology involved in the
study of pathway participation in both intact and cell-free biological
systems. Here one finds that most of the reported methods involve two
different approaches: (a) the detection of individual pathways, and (b)
the estimation of pathway participation, analogous to the qualitative
and quantitative analysis of a given problem. In the former, demonstration of the presence of individual enzymes associated with a pathway in
a given biological system often is taken as presumptive evidence of the
operation of the pathway in question. Details in this regard have been
given in the preceding section. However, it should be stressed that the
occurrence of a given pathway represents the sequential operation of a