9. COMPARATIVE BIOCHEMISTRY OF GLYCOLYSIS
425
of the Embden-Meyerhof pathway as yet is unknown in most tissues
because of the almost ubiquitous occurrence of the alternate pentose
phosphate shunt. The problem of interpreting comparative results of
the initial oxidation of various labeled glucose and other hexose molecules has been one of the barriers to final elucidation of this important
problem of cellular metabolism (93, 94).
Another aspect of glycolysis which also needs clarification is its possible importance in those metabolic activities of the cell not involved
directly with energy production. For example, it is considered probable
that the degradation of glucose-6-phosphate by the pentose phosphate
pathway is important in the formation of reduced TPN for some synthetic reactions (209-213a). Does the generation of reduced DPN by
the oxidative step in the glycolytic scheme have any importance in cell
metabolism other than in mitochondrial oxidative phosphorylation? Does
the apparent intracellular localization of the enzymes of both glycolysis
(214) and the hexose phosphate pathway (215) in the nonparticulate
portions of the cell have any special functional significance in providing
both reduced pyridine nucleotide coenzymes for synthesis and for other
special activities of cell particulates?
Despite the uncertainties concerned with the metabolic role of
glycolysis in most animal tissues, there are a few types of cells in which
the position of glycolysis is somewhat better understood. Skeletal muscle
probably is the best example (216). This is one of the few tissues of
higher animals capable of incurring a significant oxygen debt. In muscle,
glycolysis is capable of generating sufficient ATP for replenishment of
the energy stores. However, even here, glycolysis by itself is only effective for relatively short periods of time.
Another interesting example of the role of glycolysis in cell physiology is found in mammalian spermatozoa (217). In many species, these
cells show a variant of the classical glycolysis in that the predominant
primary substrate for glycolysis is fructose, rather than glucose or glycogen (218, 219). Apparently, spermatozoan motility and function can
be maintained in large measure by the glycolytic degradation of fructose,
called fructolysis, by the Embden-Meyerhof pathway from fructose-6phosphate to lactic acid. Although glucose is metabolized as easily as is
fructose, the physiological substrate appears to be fructose. In the testis
and epidydymis, where apparently fructose is not available, the sperm
are immobile. However, they become motile on their passage down to
the urethra, where fructose is added to the seminal fluid from the accessory organs of reproduction, such as the seminal vesicles. Spermatozoa in common with most other tissues, exhibit a Pasteur effect.
Mammalian erythrocytes also appear to use glycolysis as an important
425
of the Embden-Meyerhof pathway as yet is unknown in most tissues
because of the almost ubiquitous occurrence of the alternate pentose
phosphate shunt. The problem of interpreting comparative results of
the initial oxidation of various labeled glucose and other hexose molecules has been one of the barriers to final elucidation of this important
problem of cellular metabolism (93, 94).
Another aspect of glycolysis which also needs clarification is its possible importance in those metabolic activities of the cell not involved
directly with energy production. For example, it is considered probable
that the degradation of glucose-6-phosphate by the pentose phosphate
pathway is important in the formation of reduced TPN for some synthetic reactions (209-213a). Does the generation of reduced DPN by
the oxidative step in the glycolytic scheme have any importance in cell
metabolism other than in mitochondrial oxidative phosphorylation? Does
the apparent intracellular localization of the enzymes of both glycolysis
(214) and the hexose phosphate pathway (215) in the nonparticulate
portions of the cell have any special functional significance in providing
both reduced pyridine nucleotide coenzymes for synthesis and for other
special activities of cell particulates?
Despite the uncertainties concerned with the metabolic role of
glycolysis in most animal tissues, there are a few types of cells in which
the position of glycolysis is somewhat better understood. Skeletal muscle
probably is the best example (216). This is one of the few tissues of
higher animals capable of incurring a significant oxygen debt. In muscle,
glycolysis is capable of generating sufficient ATP for replenishment of
the energy stores. However, even here, glycolysis by itself is only effective for relatively short periods of time.
Another interesting example of the role of glycolysis in cell physiology is found in mammalian spermatozoa (217). In many species, these
cells show a variant of the classical glycolysis in that the predominant
primary substrate for glycolysis is fructose, rather than glucose or glycogen (218, 219). Apparently, spermatozoan motility and function can
be maintained in large measure by the glycolytic degradation of fructose,
called fructolysis, by the Embden-Meyerhof pathway from fructose-6phosphate to lactic acid. Although glucose is metabolized as easily as is
fructose, the physiological substrate appears to be fructose. In the testis
and epidydymis, where apparently fructose is not available, the sperm
are immobile. However, they become motile on their passage down to
the urethra, where fructose is added to the seminal fluid from the accessory organs of reproduction, such as the seminal vesicles. Spermatozoa in common with most other tissues, exhibit a Pasteur effect.
Mammalian erythrocytes also appear to use glycolysis as an important