Amino Acid Sequences Containing Cysteine or Half-Cystine Residues in ß-Glucuronidase

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Abstract
Amino acid analysis of oxidized or reduced and carboxymethylated [rat] .beta.-glucuronidase show the presence of 24 cysteic acid or S-carboxymethylcysteine residues, respectively/mol of the tetrameric enzyme. Titration of SH groups gave 8 cysteine residues, and by difference 16 half-cystine residues/mol. Six peptides containing radiolabeled cysteine residues were isolated from pepsin and chymotrypsin digests of reduced and S-carboxymethylated .beta.-glucuronidase by ion-exchange chromatography or gel filtration, followed by paper ionophoresis and paper chromatography. The peptides were analyzed for amino acids and sequenced by the dansyl-Edman procedure. Peptides containing cysteic acid were selectively recovered from thermolysin digests of performic acid-oxidized glucuronidase. The amino acid sequences confirmed that there were only 6 different peptide sequences containing either cysteine or half-cystine residues in the tetrameric enzyme, supporting the presence of 4 identical subunits.