Enzyme modification by MPEG with an amino acid or peptide as spacer arms

A method for the modification of enzymes by MPEG carrying an amino acid or peptide as a spacer arm is described and tested with aliphatic or aromatic side chains amino acids. The procedure involves MPEG activation byp-nitrophenylchloroformate for the amino acid or peptide coupling that is in turn activated for the protein binding. The advantage of the method resides in the possibility to introduce proper reporter groups between the polymer and the protein as norleucine for a direct evaluation of the bound polymer chains, tryptophan for structural studies of the polymer-protein adduct, and radioactive amino acid for pharmacokinetic investigations. The method was positively tested with arginase, ribonuclease, and superoxide dismutase as enzymes of therapeutic value.