Purification and characterisation of a novel papain inhibitor from common bean (Phaseolus vulgaris) seed

A proteinaceous inhibitor of papain was purified to apparent homogeneity fr om mature seeds of common bean (Phaseolus vulgaris L.). After four chromato graphic steps, the papain inhibitor was purified 219-fold with 12% recovery . On the basis of papain inhibitory activity, cystatins have been estimated to account for about 0.1%, of the total protein content of mature common b ean seeds. The purified protein, as other plant cystatins, is an acidic pro tein, heat stable and insensitive to reducing agents. Its molecular mass is about 37 kDa as judged by size exclusion chromatography and SDS-PAGE. More over it is immunologically related to oryzacystatins, since it is recognise d by a specific oryzacystatin I antiserum. Based on its biochemical propert ies the papain inhibitor described here belongs to the phytocystatin family . Papain inhibitory assays carried out during seed development showed that bean cystatin is active since early maturation stages. Our results suggest that, in common bean seed. cysteine proteinase inhibitors are important dur ing seed development with a putative role in the control and regulation of endogenous thiol protease activity.