Sulphide formation from cysteine

Hydrogen sulphide is a key intermediate in the formation of many heterocyclic sulphur compounds. It is produced from cysteine by hydrolysis or by Strecker degradation ammonia, acetaldehyde and mercaptoacetaldehyde are also formed (Scheme 12.4). All of these are reactive compounds, providing an important source of reactants for a wide range of flavour compounds. Scheme 12.6 summarises the reactions between hydrogen sulphide and other simple intermediates formed in other parts of the Maillard reaction. 

The basic reaction of amino acids under irradiation conditions is the formation of a radical intermediate on the a-carbon atom. From this, ketoacids, iminoacids and degradation products are formed (Scheme 7.3). Cysteine is decomposed to cystine, with hydrogen sulphide formation causing the off flavour of irradiated protein-rich products. 

Spinach contains two enzymes catalysing the sulphuration of O-acetylserine. One enzyme possesses the additional ability to produce S-methyl-L-cysteine (4) or S-ethyl-L-cysteine from 0-acetyl-L-serine and methanethiol or ethanethiol, respectively. The second enzyme also catalyses these processes but, in addition, catalyses the formation of L-methionine, L-ethionine, and L-homocysteine from 0-acetyl-L-homoserine and methanethiol, ethanethiol, and sulphide, respectively. Enzymes from turnip and kidney beans are able to catalyse the formation of S-methylcysteine from O-acetylserine and methanethiol C-labelled serine, when fed to Allium cepa and A. sativum along with thiols, is converted into S-substituted cysteines. ... 

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