Abstract: Addition compounds have the formula ##STR1## wherein R.sub.1 represents an aliphatic oxycarbonyl group, benzyloxycarbonyl group which can have nuclear substituents or benzoyl, aromatic sulfonyl or aromatic sulfinyl group; R.sub.2 represents methyl, isopropyl, isobutyl, isoamyl or benzyl group; R.sub.3 represents a lower alkoxyl, benzyloxy or benzhydryloxy group and n represents 1 or 2.The addition compounds are produced by reacting an N-substituted monoaminodicarboxylic acid having the formula ##STR2## with an amino carboxlic acid ester having the formula in an aqueous medium in the presence of a protease and reacting the resulting dipeptide ester with the amino carboxylic acid ester and separating the addition compound.The addition compounds can be decomposed into the constituent dipeptide esters and amino acid esters by the action of an acid.

Abstract: Addition compounds having the formula ##STR1## wherein R.sub.1 represents an aliphatic oxycarbonyl group, benzyloxycarbonyl group which can have nuclear substituents, or benzoyl, aromatic sulfonyl or aromatic sulfinyl group; R.sub.2 represents methyl, isopropyl, isobutyl, isoamyl or benzyl group; R.sub.3 represents a lower alkoxyl, benzyloxy or benzhydryloxy group and n represents 1 or 2 are produced by reacting an N-substituted monoaminodicarboxylic acid having the formula ##STR2## with an amino carboxylic acid ester having the formula ##STR3## in the presence of a protease.

Abstract: Protease is recovered by reacting a first amino acid whose amino group is protected with a protective group with a second amino acid whose carboxyl group is protected with a protective group, in an aqueous medium in the presence of a protease to result a peptide synthesis to deposit the addition compound of a dipeptide and said second C-terminal protected amino acid; dissolving said addition compound by adding an organic solvent to a precipitate separated from the reaction mixture and isolating the protease from an organic solvent suspension or dissolving said addition compound by adding a water immiscible organic solvent and separating the organic solvent phase by a liquid-liquid separation.The addition compound can be dissolved in an organic solvent after separating the precipitate from the reaction mixture.The addition compound can be also dissolved by adding a water-immiscible organic solvent to the reaction mixture and the organic solvent phase can be separated from the aqueous phase.

Abstract: Protease is recovered by reacting a first amino acid whose amino group is protected with a protective group with a second amino acid whose carboxyl group is protected with a protective group, in an aqueous medium in the presence of a protease to result a peptide synthesis to deposit the addition compound of a dipeptide and said second C-terminal protected amino acid; dissolving said addition compound into the aqueous medium by adding a polar organic solvent which is miscible with water and separating an insoluble material from the resulting suspension by a solid-liquid separation to isolate the protease.

Abstract: Addition compounds having the formula ##STR1## wherein R.sub.1 represents an aliphatic oxycarbonyl group, benzyloxycarbonyl group which can have nuclear substituents, or benzoyl, aromatic sulfonyl or aromatic sulfinyl group; R.sub.2 represents methyl, isopropyl, isobutyl, isoamyl or benzyl group; R.sub.3 represents a lower alkoxyl, benzyloxy or benzhydryloxy group and n represents 1 or 2 together with methods for preparing (I) and for decomposing (I) into the constituent dipeptide esters and amino acid esters.