5. ONIUM COMPOUNDS
199
Unlike blood, semen carries ergothioneine as an extracellular constituent
in amounts between 30 and 250 mg./100 ml.
Although the efforts of earlier investigators (72-74) to demonstrate
the presence of ergothioneine in a variety of plant materials yielded
negative results, recently ergothioneine was found in oats (75) and it
may be presumed that it is also present in other cereal grains. This
discovery together with the earlier conclusion of the Cornell school
that blood ergothioneine must be largely of dietary origin, since no
evidence for the biosynthesis of ergothioneine by any animal species
had been found (76, 77), clarifies considerably the question of the
origin of ergothioneine in the blood of herbivorous vertebrates.
The problem has been clarified further by the finding that ergothioneine is a constituent of many species of microorganisms and that
it is synthesized by several fungi (78). It is not yet clear whether the
cereals in which ergothioneine has been found possess the ability to
synthesize it or whether ergothioneine is derived from saprophytic fungi
commonly present in cereals. The ergothioneine content of fungi grown
on chemically defined media varies with the species, ranging between
4 and 85 mg. per 100 gm. of dried cells. Although many bacterial cells
contain ergothioneine, no bacterial species is capable of synthesizing
the compound (78). It has been shown that the presence of ergothioneine in the bacterial cell is due to the assimilation of this betaine
from the growth medium. Some species, notably Micrococous pyogenes
accumulate it from the medium in unusually large amounts and concentrate it inside the cell to levels of 50 mg. per 100 gm. of dry cells.
b. Biosynthesis. The biochemical pathway and the precursors involved in the biosynthesis of this thiolbetaine has been elucidated in
the main outlines by the work of Melville et al. (79) and of Heath
et al. (80-82). It has thus become established that three amino acids,
histidine, cysteine, and methionine, contribute to the different moieties
of the ergothioneine molecule. Specifically, histidine supplies the imidazole nucleus and the side chain, while the sulfhydryl group is derived from cysteine and methionine contributes one or possibly all of
the three methyl groups. The evidence suggests that the imidazole ring
of histidine is not degraded in the conversion to ergothioneine and is
incorporated intact. Thiolhistidine failed to serve effectively as a precursor of ergothioneine in Neurospora crassa (79), but this is not due
to the inability of thiolhistidine to enter the cells. This would signify
then that thiolhistidine is not directly on the synthetic pathway, and
that methylation of histidine, either partial or complete, must precede
the introduction of the sulfhydryl group. This is in agreement with the
fact that thiolhistidine has never been shown to occur naturally,
199
Unlike blood, semen carries ergothioneine as an extracellular constituent
in amounts between 30 and 250 mg./100 ml.
Although the efforts of earlier investigators (72-74) to demonstrate
the presence of ergothioneine in a variety of plant materials yielded
negative results, recently ergothioneine was found in oats (75) and it
may be presumed that it is also present in other cereal grains. This
discovery together with the earlier conclusion of the Cornell school
that blood ergothioneine must be largely of dietary origin, since no
evidence for the biosynthesis of ergothioneine by any animal species
had been found (76, 77), clarifies considerably the question of the
origin of ergothioneine in the blood of herbivorous vertebrates.
The problem has been clarified further by the finding that ergothioneine is a constituent of many species of microorganisms and that
it is synthesized by several fungi (78). It is not yet clear whether the
cereals in which ergothioneine has been found possess the ability to
synthesize it or whether ergothioneine is derived from saprophytic fungi
commonly present in cereals. The ergothioneine content of fungi grown
on chemically defined media varies with the species, ranging between
4 and 85 mg. per 100 gm. of dried cells. Although many bacterial cells
contain ergothioneine, no bacterial species is capable of synthesizing
the compound (78). It has been shown that the presence of ergothioneine in the bacterial cell is due to the assimilation of this betaine
from the growth medium. Some species, notably Micrococous pyogenes
accumulate it from the medium in unusually large amounts and concentrate it inside the cell to levels of 50 mg. per 100 gm. of dry cells.
b. Biosynthesis. The biochemical pathway and the precursors involved in the biosynthesis of this thiolbetaine has been elucidated in
the main outlines by the work of Melville et al. (79) and of Heath
et al. (80-82). It has thus become established that three amino acids,
histidine, cysteine, and methionine, contribute to the different moieties
of the ergothioneine molecule. Specifically, histidine supplies the imidazole nucleus and the side chain, while the sulfhydryl group is derived from cysteine and methionine contributes one or possibly all of
the three methyl groups. The evidence suggests that the imidazole ring
of histidine is not degraded in the conversion to ergothioneine and is
incorporated intact. Thiolhistidine failed to serve effectively as a precursor of ergothioneine in Neurospora crassa (79), but this is not due
to the inability of thiolhistidine to enter the cells. This would signify
then that thiolhistidine is not directly on the synthetic pathway, and
that methylation of histidine, either partial or complete, must precede
the introduction of the sulfhydryl group. This is in agreement with the
fact that thiolhistidine has never been shown to occur naturally,