The present invention concerns the field of conjugated peptides suitable for the production of drugs having an improved plasma half-life. In particular the present invention relates to a conjugated protein, obtained by an enzymatic reaction via microbial transglutaminase (MTGase), and an improved process for removing residual transglutaminase from peptides or recombinant proteins enzymatically conjugated by microbial transglutaminase (MTGase) to hydrophilic non-immunogenic polymer at a glutamine side-chain through an amidic linkage, allowing to obtain purified conjugated peptides or proteins which are stable against the enzymatic hydrolysis of the amidic bond between the peptide or protein moiety and the hydrophilic polymer and being free from product derived degradation displays the stability required for a drug.
