9. COMPARATIVE BIOCHEMISTRY OF GLYCOLYSIS
421
equinum, T. evansi) which have a high rate of carbohydrate utilization
is not cyanide-sensitive (143, 144, 148, 150, 151); (b) the respiratory
quotient of the latter organisms is extremely low (146, 151, 152) while
it is high in T. cruzi (144, 153) and in T. lewisi (144, 146, 154, 155),
suggesting the occurrence of oxidative decarboxylations in the latter
group of organisms. It is noteworthy that in these respects T. congolense
occupies a somewhat intermediate position between these two groups
(150, 151, 156) and that these biochemical differentiations closely
parallel the morphological and taxonomic classification of trypanosomes (157).
Another group of protozoa in which the Embden-Meyerhof scheme
is operating at a high rate are the erythrocytic stages of malarial parasites (158). Anaerobically, these organisms, in contrast to trypanosomes,
quantitatively convert glucose to lactic acid (159-162). Glucose, as well
as lactic acid, are oxidized completely to carbon dioxide and water;
since the rate of glycolysis of malarial parasites is considerably higher
than that of carbohydrate oxidation, lactic acid accumulates even under
aerobic conditions; thus, a considerable amount of energy, generated
aerobically, originates from glycolytic reactions. Yet, the organisms do
not multiply under completely anaerobic conditions (163); therefore, at
least some of the energy required for reproduction of malarial parasites
cannot be supplied by glycolysis.
While the occurrence and the role of glycolysis has been demonstrated in many protozoa, little information is available about alternate
pathways of glucose utilization in these organisms. Recently it has been
reported that formation of glyceraldehyde-3-phosphate and of pyruvate
from phosphogluconate and from 2-keto-3-desoxy-6-phosphogluconate
probably represents the major pathway of glucose utilization of
Endamoeba histolytica (164). Therefore, the existence of this pathway
is not restricted to a relatively limited number of bacteria (pseudomonads, aerobacters).
D. HELMINTHS
A common characteristic of these invertebrate metazoa is a high rate
of carbohydrate metabolism, associated with incomplete substrate oxidation. This not only applies to intestinal helminths whose habitat is
nearly anaerobic, but also to parasitic worms, such as schistosomes,
which live in oxygen-rich surroundings. Although helminths take up
oxygen when it is available to them, fermentation rates remain high,
even under aerobic conditions. Many of these organisms depend for
survival on anaerobic metabolism and appear to be unable to utilize
energy derived from oxidative reactions (145, 165).
421
equinum, T. evansi) which have a high rate of carbohydrate utilization
is not cyanide-sensitive (143, 144, 148, 150, 151); (b) the respiratory
quotient of the latter organisms is extremely low (146, 151, 152) while
it is high in T. cruzi (144, 153) and in T. lewisi (144, 146, 154, 155),
suggesting the occurrence of oxidative decarboxylations in the latter
group of organisms. It is noteworthy that in these respects T. congolense
occupies a somewhat intermediate position between these two groups
(150, 151, 156) and that these biochemical differentiations closely
parallel the morphological and taxonomic classification of trypanosomes (157).
Another group of protozoa in which the Embden-Meyerhof scheme
is operating at a high rate are the erythrocytic stages of malarial parasites (158). Anaerobically, these organisms, in contrast to trypanosomes,
quantitatively convert glucose to lactic acid (159-162). Glucose, as well
as lactic acid, are oxidized completely to carbon dioxide and water;
since the rate of glycolysis of malarial parasites is considerably higher
than that of carbohydrate oxidation, lactic acid accumulates even under
aerobic conditions; thus, a considerable amount of energy, generated
aerobically, originates from glycolytic reactions. Yet, the organisms do
not multiply under completely anaerobic conditions (163); therefore, at
least some of the energy required for reproduction of malarial parasites
cannot be supplied by glycolysis.
While the occurrence and the role of glycolysis has been demonstrated in many protozoa, little information is available about alternate
pathways of glucose utilization in these organisms. Recently it has been
reported that formation of glyceraldehyde-3-phosphate and of pyruvate
from phosphogluconate and from 2-keto-3-desoxy-6-phosphogluconate
probably represents the major pathway of glucose utilization of
Endamoeba histolytica (164). Therefore, the existence of this pathway
is not restricted to a relatively limited number of bacteria (pseudomonads, aerobacters).
D. HELMINTHS
A common characteristic of these invertebrate metazoa is a high rate
of carbohydrate metabolism, associated with incomplete substrate oxidation. This not only applies to intestinal helminths whose habitat is
nearly anaerobic, but also to parasitic worms, such as schistosomes,
which live in oxygen-rich surroundings. Although helminths take up
oxygen when it is available to them, fermentation rates remain high,
even under aerobic conditions. Many of these organisms depend for
survival on anaerobic metabolism and appear to be unable to utilize
energy derived from oxidative reactions (145, 165).