9. COMPARATIVE BIOCHEMISTRY OF GLYCOLYSIS
423
This hypothesis is supported by the finding that Ascaris muscle contains enzyme systems which catalyze the reduction of α-acetolactate and
of a-methylacetoacetate by DPNH (175). The activities of these reducing systems are considerably higher than that of lactic dehydrogenase of Ascaris muscle (184). The formation of considerable quantities of succinate by Ascaris (173) and by other helminths (170, 171)
also could originate with pyruvate and may proceed through C0 2 fixation, catalyzed by the "malic" enzyme, followed by a conversion of
malate to succinate via fumarate. Such a reaction sequence would be
consistent with the high "malic" enzyme (185), fumarase (185), and
succinic dehydrogenase (186, 187) activities of Ascaris muscle.
An increased energy requirement produces a shift from fatty acid to
lactic acid fermentation in the liver fluke Fasciola hepatica. The major
products of carbohydrate metabolism of this helminth are propionate
and acetate (in an approximate ratio of 3:1) while only 4 to 8% of the
utilized carbohydrate is converted to lactate (166). Stimulation of the
muscular activity of the parasite by low concentration of 5-hydroxytryptamine or of lysergic acid diethylamide results in a two- to sixfold increase in lactate production while there is little or no change in propionate or acetate formation (166, 188). It would appear that in these
organisms lactic acid fermentation can meet increased energy requirements more efficiently than fatty acid fermentation.
E. INSECTS
An interesting variant of glycolysis has been described recently in
the extremely active flight muscle of the housefly (Musca domestica)
(189) and of some Hymenoptera. In these species, lactic dehydrogenase activity is of a very low order when compared with crayfish or
rat muscle; on the other hand, the activity of «-glycerophosphate dehydrogenase is very high (190, 191). All the other glycolytic enzymes
concerned with the production of pyruvic acid from hexoses are present
in these insect flight muscles (192, 193) as well as enzymes of the pentose phosphate pathway (194, 195) and of the tricarboxylic acid cycle
(193, 196). Unlike the muscle of higher animals, lactic acid is oxidized
slowly, if at all, in fly flight muscle preparations (193). Glycerophosphate
has been found to accumulate in insect muscle (197).
On the basis of these and other findings, including the discovery of a
very active non-pyridine-linked glycerophosphate dehydrogenase in flight
muscle mitochondria (190, 193, 198-201), Sacktor has suggested that
glycogen, glucose, and several other sugars are metabolized to pyruvic
acid via the Embden-Meyerhof scheme present in the sarcoplasm or
nonparticulate part of the muscle (189). However, DPNH, produced
423
This hypothesis is supported by the finding that Ascaris muscle contains enzyme systems which catalyze the reduction of α-acetolactate and
of a-methylacetoacetate by DPNH (175). The activities of these reducing systems are considerably higher than that of lactic dehydrogenase of Ascaris muscle (184). The formation of considerable quantities of succinate by Ascaris (173) and by other helminths (170, 171)
also could originate with pyruvate and may proceed through C0 2 fixation, catalyzed by the "malic" enzyme, followed by a conversion of
malate to succinate via fumarate. Such a reaction sequence would be
consistent with the high "malic" enzyme (185), fumarase (185), and
succinic dehydrogenase (186, 187) activities of Ascaris muscle.
An increased energy requirement produces a shift from fatty acid to
lactic acid fermentation in the liver fluke Fasciola hepatica. The major
products of carbohydrate metabolism of this helminth are propionate
and acetate (in an approximate ratio of 3:1) while only 4 to 8% of the
utilized carbohydrate is converted to lactate (166). Stimulation of the
muscular activity of the parasite by low concentration of 5-hydroxytryptamine or of lysergic acid diethylamide results in a two- to sixfold increase in lactate production while there is little or no change in propionate or acetate formation (166, 188). It would appear that in these
organisms lactic acid fermentation can meet increased energy requirements more efficiently than fatty acid fermentation.
E. INSECTS
An interesting variant of glycolysis has been described recently in
the extremely active flight muscle of the housefly (Musca domestica)
(189) and of some Hymenoptera. In these species, lactic dehydrogenase activity is of a very low order when compared with crayfish or
rat muscle; on the other hand, the activity of «-glycerophosphate dehydrogenase is very high (190, 191). All the other glycolytic enzymes
concerned with the production of pyruvic acid from hexoses are present
in these insect flight muscles (192, 193) as well as enzymes of the pentose phosphate pathway (194, 195) and of the tricarboxylic acid cycle
(193, 196). Unlike the muscle of higher animals, lactic acid is oxidized
slowly, if at all, in fly flight muscle preparations (193). Glycerophosphate
has been found to accumulate in insect muscle (197).
On the basis of these and other findings, including the discovery of a
very active non-pyridine-linked glycerophosphate dehydrogenase in flight
muscle mitochondria (190, 193, 198-201), Sacktor has suggested that
glycogen, glucose, and several other sugars are metabolized to pyruvic
acid via the Embden-Meyerhof scheme present in the sarcoplasm or
nonparticulate part of the muscle (189). However, DPNH, produced