Mono-pegylated arginase conjugate and method producing thereof. The mono-pegylated arginase is homogeneous in molecular weight and shows therapeutic effect for treating cancers and viral infections. The method of producing such arginase conjugate has a main step of genetically modifying the gene encoding an arginase so that the PEG moiety can attach to the enzyme at a predetermined, specific intended site. This is achieved by removing the PEG attaching amino acid residues at undesirable sites while keeping (or adding, if necessary) the one at the desirable site of the enzyme. Two exemplary mono-pegylated arginase conjugates so produced are human arginase I (HAI) where a polyethylene glycol (PEG) moiety is site-specific covalently bonded to Cys45 of the enzyme and Bacillus caldovelox arginase (BCA) where a polyethylene glycol (PEG) moiety is site-specific covalently bonded to Cys161 of the enzyme.
