Abstract Sodium salts of methyl esters of N-tert-butyloxycarbonyl-β-tert-butylaspartyl-O 4- sulfotyrosine (II) or N-tert-butyloxycarbonyl-O 4-sulfotyrosine (III) were condensed with amino components derived from peptide amides IVb-IVe and IVg (simulating the carboxy- terminal part of cholecystokinin) under catalysis with papain. Rates and yields of conversion of these peptides to the corresponding derivatives Ib-If were compared with the results ...