Methylcysteine

A review of the early 1970 s details the manganese compounds of sulfur-containing amino acids, and quotes evidence that cysteine (172 R = SH) and penicillamine bind via both the N and S donor atoms at higher pH values however, analysis of the papers quoted yielded little evidence for S binding. On the other hand, there is sound evidence that when the thiol group is methylated, as in 5-methylcysteine and methionine (172 R = SMe and CHjSMe respectively), the S does not coordinate, either in solution or in the solid state. ... 

In the nonprotein fraction reduced glutathione, GSH, is ubiquitous, and is commonly a mqjor constituent (Table I). The soluble fraction of plants also includes a variety of other sulfur-containing compounds that are normally present in relatively small amounts (a) Intermediates on the route to protein cysteine and protein methionine, such as cysteine, cystathionine, homocysteine, and methionine, (b) Compounds involved in methyl transfer reactions and polyamine synthesis AdoMet.t AdoHcy, and, presumably, 5 -methyl-thioadenosine. The biochemistry of the compounds in both groups (a) and (b) will be discussed here, (c) Compounds clearly related metabolically to cysteine or methionine, such as 5-methylcysteine and 5-methylmethionine. Because in certain plants these derivatives comprise a major portion of the nonprotein sulfur amino acids, they will be discussed here, (d) A number of compounds of uncertain function, the biochemistry of which has often not been clarified. Discussion of such compounds (Richmond, 1973 Fowden, 1964) is beyond the scope of this chapter. 

Cystathionine-/3-lyase has been purified 430-fold from spinach leaves (Giovanelli and Mudd, 1971). The enzyme contains a firmly bound pyridoxal phosphate prosthetic group, and was isolated as the holoenzyme. The substrate specificity of the plant enzyme resembles that of the bacterial enzyme, being most active with cystathionine and djenkolate, and far less active with cystine, 5-methylcysteine, and serine, but differs from that of the fungal enzyme, the activity of which with cysteine, serine, and lanthionine is about the same as that with cystathionine. Conversely, with respect to sensitivity to inhibition by 5, 5 -dithiobis-2-nitrobenzoic acid and A-ethylmaleimide, the plant enzyme resembles the fungal enzyme. 

Two pathways for the de novo synthesis of 5-methylcysteine have been proposed. The first is by incorporation of the methylthio moiety as a unit. An enzyme activity capable of explaining such an incorporation has been detected in extracts of spinach (Giovanelli and Mudd, 1968), turnip leaves (Thompson and Moore, 1968) and onion (Granroth and Samesto, 1974). 

Methylcysteine sulfoxide accumulates in certain plants of the Cruciferae and Liliaceae (Thompson, 1967), but could not be detected in several legumes or plants of six other families examined (Fujiwaraet ai, 1958). The compound has been isolated exclusively as (-t-XS-methyl-L-cysteine sulfoxide, suggesting that it is not formed as an artifact by chemical oxidation of 5-methylcysteine. 

Reduction of 5-methylcysteine sulfoxide to 5-methylcysteine has been demonstrated in the leaves of turnip and bean (Doney and Thompson, 1966). In turnip, the (-)diastereoisomer was reduced more rapidly than the (+)diastereoisomer, while in bean the opposite stereospecificity was observed. 

The enzymatic )8-cleavage of 5-methylcysteine and its sulfoxide is discussed by Mazelis (this volume. Chapter 15). 

During germination of kidney beans, in which 5-methylcysteine is present mainly in the form of the y-glutamyl derivative, there is a net loss of over 70% of the total -y-glutamyl-5-methylcysteine. Since no increase in acidic peptides equivalent to the net loss of y-glutamyl-5-methylcysteine could be detected, it was concluded that hydrolysis rather than transpeptidation had occurred, and it was suggested (Thompson et al., 1962) that y-... 

Although these combined observations are suggestive, the function and physiological significance of 5-methylcysteine and its derivatives in plants remain to be established. 

Several polysulphides occur in hop oil. Dimethyl trisulphide (2,3,4-trithiapentane) occurs only in oil prepared from hops which have not been treated with sulphur (dioxide) on the kiln. It is absent from oil prepared by vacuum distillation at 25°C but is formed at lOO C from the precursor 5-methylcysteine sulphoxide (CH3SO-CH2CH(NH2)COOH). This precursor is destroyed by sulphur dioxide when sulphur is burnt in the oast but is slowly regenerated on storage. Dimethyl tetrasulphide and 2,3,5-trithiahexane are... 

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