5. ONIUM COMPOUNDS
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3. S-Methylmethionine
S-Methylmethionine, the third naturally occurring sulfonium compound, has so far been found only in plants. It was first isolated by
McRorie et al. (151) who identified this analog of methionine with a
natural material found in cabbage juice which is capable of counteracting the toxicity of sulfanilamide in E. coli (152). It is interesting to
point out that under conditions of limiting methionine biosynthesis
S-methylmethionine is 2-7 times more active than molar equivalents of
methionine in reversing sulfanilamide inhibition. This observation suggests that S-methylmethionine may serve not only as a precursor of
methionine but also as a methyl donor or as a precursor of some essential cell constituent, possibly adenosylmethionine as was just noted for
yeast. In cabbage S-methylmethionine is found in concentrations similar
to those quoted for S-adenosylmethionine in yeast (2 /xmoles per gram
dry weight). S-methylmethionine is found in a variety of other plants,
such as (named in the relative order of concentrations) parsley, turnip
greens, turnip, pepper, carrot, onions, lettuce (151), and asparagus
(153).
As would be expected the chemical properties of S-methylmethionine
resemble those of adenosylmethionine and of dimethylpropiothetin. It
is quite stable to acid; at pH 7 or higher it decomposes rapidly to dimethylsulfide and 2-amino-4-butyrolactone, which then hydrates to
homoserine.
Little is known about the biological origin and function of S-methylmethionine. As indicated, it is able to replace methionine both in microorganisms and in vertebrates (154), and in fact, under some conditions it is superior to methionine as a growth factor for methionine
auxothrophs of E. coli, L. arabinosus, and L. casei, Proteus, and Aerobacter aerogenes (155, 156).*
These results, like the ones of McRorie (152) and those of Schlenk
reviewed above (141), indicate that in certain cases S-methylmethionine
is a more direct precursor of an active intermediate or an essential cell
constituent than is methionine itself, but give no clue as to the nature
of the compound involved. It is possible to visualize at least two alternative mechanisms to explain these observations. In the first place it is
now known that S-methylmethionine can transfer enzymatically one of
its methyl groups to homocysteine (157), and in this way dietary supplementation with S-methylmethionine may result in a higher intra* However, Streptococcus faecalis R, Leuconostoc mesenteroides P-60, some
strains of E. coli and of Neurospora crassa do not have the ability to utilize
S-methylmethionine in lieu of methionine.
215
3. S-Methylmethionine
S-Methylmethionine, the third naturally occurring sulfonium compound, has so far been found only in plants. It was first isolated by
McRorie et al. (151) who identified this analog of methionine with a
natural material found in cabbage juice which is capable of counteracting the toxicity of sulfanilamide in E. coli (152). It is interesting to
point out that under conditions of limiting methionine biosynthesis
S-methylmethionine is 2-7 times more active than molar equivalents of
methionine in reversing sulfanilamide inhibition. This observation suggests that S-methylmethionine may serve not only as a precursor of
methionine but also as a methyl donor or as a precursor of some essential cell constituent, possibly adenosylmethionine as was just noted for
yeast. In cabbage S-methylmethionine is found in concentrations similar
to those quoted for S-adenosylmethionine in yeast (2 /xmoles per gram
dry weight). S-methylmethionine is found in a variety of other plants,
such as (named in the relative order of concentrations) parsley, turnip
greens, turnip, pepper, carrot, onions, lettuce (151), and asparagus
(153).
As would be expected the chemical properties of S-methylmethionine
resemble those of adenosylmethionine and of dimethylpropiothetin. It
is quite stable to acid; at pH 7 or higher it decomposes rapidly to dimethylsulfide and 2-amino-4-butyrolactone, which then hydrates to
homoserine.
Little is known about the biological origin and function of S-methylmethionine. As indicated, it is able to replace methionine both in microorganisms and in vertebrates (154), and in fact, under some conditions it is superior to methionine as a growth factor for methionine
auxothrophs of E. coli, L. arabinosus, and L. casei, Proteus, and Aerobacter aerogenes (155, 156).*
These results, like the ones of McRorie (152) and those of Schlenk
reviewed above (141), indicate that in certain cases S-methylmethionine
is a more direct precursor of an active intermediate or an essential cell
constituent than is methionine itself, but give no clue as to the nature
of the compound involved. It is possible to visualize at least two alternative mechanisms to explain these observations. In the first place it is
now known that S-methylmethionine can transfer enzymatically one of
its methyl groups to homocysteine (157), and in this way dietary supplementation with S-methylmethionine may result in a higher intra* However, Streptococcus faecalis R, Leuconostoc mesenteroides P-60, some
strains of E. coli and of Neurospora crassa do not have the ability to utilize
S-methylmethionine in lieu of methionine.