Tables in section 3AA-18.1 of these recommendations give many examples of such usage, but these tables do not include examples of amides of amino acids and peptides. As such amides are often used as substrates for peptidases, symbols for them are given here as examples of the symbolism. We recommend the name 'selenocysteine' for cysteine in which sulfur is replaced by selenium. It seems reasonable to extent its usage to cover replacement of sulfur, so that the amino acid may be named 'selenocysteine', as opposed to the alternatives 'selenoserine' and 'hydroselenoalanine', which would adhere more strictly to the existing rules of organic nomenclature but would be likely to be more obscure in practice. By extension, the prefix seleno may be applied to trivially named peptides such as glutathione to represent replacement of sulfur by selenium.
Similar emphasis is used in the fourth formula.
The need for a similar symbol to indicate hydrolysis that brings about ring opening rather than creation of two molecules may also exist for homodetic cyclic peptides, i.e. peptides such as gramicidin S in which the ring consists solely of amino-acid residues in eupeptide linkage. Of the possible names that have been suggested, 'anhydroaspartic residue' seems to be the clearest concise way of indicating the type of structure. We thank Dr A. Zagari for bringing this point to our attention. References 14 . Recommendations 1983, Biochem.