9. COMPARATIVE BIOCHEMISTRY OF GLYCOLYSIS
419
in this metabolic route. Studies involving the use of glucose-1-C
14 and
of glucose-6-C
14 have revealed that in young plant tissue carbohydrate
is utilized predominantly, if not exclusively, via the Embden-Meyerhof
scheme; as the plant tissues mature the contribution of the pentose phosphate pathway becomes more and more pronounced, and in older tissues
up to 50% of the carbohydrate may be metabolized through this route
(135,137,138).
C. PROTOZOA
Photo trophic and oxy trophic* protozoa utilize endogenous polysaccharides, but are unable to metabolize exogenous carbohydrate sources.
These organisms accumulate large quantities of starch and use this
polysaccharide as a source of energy. By contrast, glucose, mannose,
and other mono- and disaccharides are not utilized by these protozoa
which contain no hexokinase. It has been demonstrated (139) that
these organisms contain a phosphorylase which catalyzes the reversible
synthesis of starch from glucose-1-phosphate. Evidently, the latter cannot originate from the phosphorylation of glucose because of the absence
of hexokinase, but formation of glucose-1-phosphate through photosynthetic mechanisms, or through utilization of acetate, lactate, or pyruvate,
involving the formation of 2-phosphoglycerate, followed by a reversal of
the Embden-Meyerhof scheme, can account for the synthesis of polysaccharide in photosynthetic and oxytrophic protozoa (139).
In contrast to these organisms, polytrophic or haplotrophic protozoa
do not store significant amounts of polysaccharide, but utilize exogenous
glucose and other carbohydrates as energy sources. Among these organisms, many trypanosomes have an exceedingly high rate of, and a marked
dependence on, carbohydrate metabolism. According to calculations of
Von Brand (140) the African pathogenic trypanosomes (Γ. gambiense,
T. rhodesiense, T. congolense, T. brucei, T. evansi, T. equiperdum, and
T. hippicum) consume in one hour an amount of glucose equivalent to
50 to 100% of their dry weight. The participation of the Embden-Meyerhof scheme in the utilization of glucose has been demonstrated in these
organisms (141-143), although only traces of lactic acid are produced
(144, 145) or none at all. Under anaerobic conditions 1 mole of glucose
gives rise to 1 mole of glycerol and 1 mole of pyruvate while aerobically
2 moles of pyruvate are formed (142-144, 146). It appears that these
organisms have very little lactic dehydrogenase or even lack this enzyme
* Oxytrophic protozoa are organisms capable of utilizing organic acids, especially
lower fatty acids, as a source of energy, but unable to utilize sugars and amino
acids. These organisms are distinguished from haplotrophic or polytrophic protozoa
whose major energy requirements are met by the metabolism of sugars (139).
419
in this metabolic route. Studies involving the use of glucose-1-C
14 and
of glucose-6-C
14 have revealed that in young plant tissue carbohydrate
is utilized predominantly, if not exclusively, via the Embden-Meyerhof
scheme; as the plant tissues mature the contribution of the pentose phosphate pathway becomes more and more pronounced, and in older tissues
up to 50% of the carbohydrate may be metabolized through this route
(135,137,138).
C. PROTOZOA
Photo trophic and oxy trophic* protozoa utilize endogenous polysaccharides, but are unable to metabolize exogenous carbohydrate sources.
These organisms accumulate large quantities of starch and use this
polysaccharide as a source of energy. By contrast, glucose, mannose,
and other mono- and disaccharides are not utilized by these protozoa
which contain no hexokinase. It has been demonstrated (139) that
these organisms contain a phosphorylase which catalyzes the reversible
synthesis of starch from glucose-1-phosphate. Evidently, the latter cannot originate from the phosphorylation of glucose because of the absence
of hexokinase, but formation of glucose-1-phosphate through photosynthetic mechanisms, or through utilization of acetate, lactate, or pyruvate,
involving the formation of 2-phosphoglycerate, followed by a reversal of
the Embden-Meyerhof scheme, can account for the synthesis of polysaccharide in photosynthetic and oxytrophic protozoa (139).
In contrast to these organisms, polytrophic or haplotrophic protozoa
do not store significant amounts of polysaccharide, but utilize exogenous
glucose and other carbohydrates as energy sources. Among these organisms, many trypanosomes have an exceedingly high rate of, and a marked
dependence on, carbohydrate metabolism. According to calculations of
Von Brand (140) the African pathogenic trypanosomes (Γ. gambiense,
T. rhodesiense, T. congolense, T. brucei, T. evansi, T. equiperdum, and
T. hippicum) consume in one hour an amount of glucose equivalent to
50 to 100% of their dry weight. The participation of the Embden-Meyerhof scheme in the utilization of glucose has been demonstrated in these
organisms (141-143), although only traces of lactic acid are produced
(144, 145) or none at all. Under anaerobic conditions 1 mole of glucose
gives rise to 1 mole of glycerol and 1 mole of pyruvate while aerobically
2 moles of pyruvate are formed (142-144, 146). It appears that these
organisms have very little lactic dehydrogenase or even lack this enzyme
* Oxytrophic protozoa are organisms capable of utilizing organic acids, especially
lower fatty acids, as a source of energy, but unable to utilize sugars and amino
acids. These organisms are distinguished from haplotrophic or polytrophic protozoa
whose major energy requirements are met by the metabolism of sugars (139).