Amino acid sequence of a peptide containing the active cysteine residue of histidine ammonia-lyase.

1. Oxidized (polymerized) histidine ammonia-lyase from Pseudomonas testosteroni was activated with dithiothreitol and the reduced disulphide-linked cysteine residues of the native enzyme were carboxymethylated with iodo[(14)C]acetate. 2. The activity of the carboxymethylated enzyme was similar to that of the polymerized form and approx. 15% of that of the fully reduced form. 3. A tryptic digest of the [(14)C]carboxymethylated enzyme contained only one radioactive peptide. 4. The amino acid sequence of this peptide was shown to be Gly-Leu-Leu-Asp-Gly-Ser-Ala-Ile-Asn-Pro-Ser-His-Pro-Asn-Cys- (CH(2)CO(2)H)-Gly-Arg. 5. These findings show that, during polymerization, the disulphide bonds are formed between identical regions of the enzyme, and that the cysteine residue involved is also the one required in the reduced state for full activity of the enzyme.