9. SACCHARIDES: ALTERNATE ROUTES OF METABOLISM
463
in Eq. 7 are applied. The data in Table VII are less certain quantitatively, hence the terms "major" or "minor" are applied as appropriate.
With regard to the catabolism of glucose and allied carbohydrates
in animals, it appears that glucose is catabolized mainly by way of
glycolysis, whereas phosphogluconate cleavage may participate to a
minor, variable extent (184). Examples may be cited of the qualitative experiments carried out by Bloom (129) with a great variety of
rabbit tissues including brain, kidney, heart, spleen, liver, bone marrow,
and testes; the estimation of glucose pathways in rat tissue (126, 127,
185), intact rats (186), mammary gland of rats (125), blood cells (3),
insects (187), cow's udder (188), retina (189), carp (190), etc. Interesting side reactions have been revealed; the occurrence of the pentose cycle as well as the extent of recycling of hexose phosphate in
cows were examined by Wood (191). By studying the isotopic distribution patterns of amino acids, glycerol (in the fat portion), and
glucose isolated from milk derived from cows fed with specifically
labeled substrate, it was possible to detect the transaldolase exchange
reaction and enable the author to examine the possible effect of side
reactions upon the estimation of pathways of glucose catabolism in
biological systems.
As discussed in Section V,B on the role of the pentose cycle, the
close relationship between the photosynthetic cycle and the pentose
cycle in plants leads one to suspect that the latter pathway should play
a role in the over-all hexose catabolism of plants. The operation of the
pentose pathway has been demonstrated in corn coleoptiles, pea seedlings, and other tissues (14, 51, 128a, 192, 193, 194) using C
14 specifically labeled hexoses. The radiorespirometric method has been used to
estimate the participation of the pentose phosphate pathway in tomato
and pepper fruits (see references 138, 139, and 195); the findings
indicated that glycolysis prevails in these fruits, with the pentose phosphate pathway operating to an extent of only 20-30%. In germinating
pea seeds (196, 197) radiorespirometric examination has revealed that
glucose is catabolized almost exclusively by glycolysis, with phosphogluconate cleavage estimated to be not more than 5% of the total
metabolized glucose. The participation of various glucose breakdown
routes in leaf tissues has not yet been thoroughly studied. From current
information, it appears that the catabolism of glucose in the plant
relies heavily on the operation of the EMP pathway. Here again
fructose-6-phosphate can be derived from glucose by way of the pentose
cycle reactions; however, judging from the findings with tomato fruit
utilizing C
14 specifically labeled gluconate (139), the regenerated fructoses-phosphate may have participated in further catabolism in a man-
463
in Eq. 7 are applied. The data in Table VII are less certain quantitatively, hence the terms "major" or "minor" are applied as appropriate.
With regard to the catabolism of glucose and allied carbohydrates
in animals, it appears that glucose is catabolized mainly by way of
glycolysis, whereas phosphogluconate cleavage may participate to a
minor, variable extent (184). Examples may be cited of the qualitative experiments carried out by Bloom (129) with a great variety of
rabbit tissues including brain, kidney, heart, spleen, liver, bone marrow,
and testes; the estimation of glucose pathways in rat tissue (126, 127,
185), intact rats (186), mammary gland of rats (125), blood cells (3),
insects (187), cow's udder (188), retina (189), carp (190), etc. Interesting side reactions have been revealed; the occurrence of the pentose cycle as well as the extent of recycling of hexose phosphate in
cows were examined by Wood (191). By studying the isotopic distribution patterns of amino acids, glycerol (in the fat portion), and
glucose isolated from milk derived from cows fed with specifically
labeled substrate, it was possible to detect the transaldolase exchange
reaction and enable the author to examine the possible effect of side
reactions upon the estimation of pathways of glucose catabolism in
biological systems.
As discussed in Section V,B on the role of the pentose cycle, the
close relationship between the photosynthetic cycle and the pentose
cycle in plants leads one to suspect that the latter pathway should play
a role in the over-all hexose catabolism of plants. The operation of the
pentose pathway has been demonstrated in corn coleoptiles, pea seedlings, and other tissues (14, 51, 128a, 192, 193, 194) using C
14 specifically labeled hexoses. The radiorespirometric method has been used to
estimate the participation of the pentose phosphate pathway in tomato
and pepper fruits (see references 138, 139, and 195); the findings
indicated that glycolysis prevails in these fruits, with the pentose phosphate pathway operating to an extent of only 20-30%. In germinating
pea seeds (196, 197) radiorespirometric examination has revealed that
glucose is catabolized almost exclusively by glycolysis, with phosphogluconate cleavage estimated to be not more than 5% of the total
metabolized glucose. The participation of various glucose breakdown
routes in leaf tissues has not yet been thoroughly studied. From current
information, it appears that the catabolism of glucose in the plant
relies heavily on the operation of the EMP pathway. Here again
fructose-6-phosphate can be derived from glucose by way of the pentose
cycle reactions; however, judging from the findings with tomato fruit
utilizing C
14 specifically labeled gluconate (139), the regenerated fructoses-phosphate may have participated in further catabolism in a man-