Methods of stabilizing proteins having at least one oxidizable amino acid thereon, such as recombinant human adenosine deaminase (rhADA) are disclosed. The methods include treating a protein containing an oxidizable amino acid with a sufficient amount of a capping agent such as glutathione under reaction conditions sufficient to cap the oxidizable amino acid without substantially inactivating the protein. Stabilized capped proteins, polymer conjugates of capped proteins, and methods of treatment using the same are also disclosed.
