The pI4.7, 14.5 kDa hematoxylin-stainable protein (HSP) from rat epidermis inhibited the activities of the cysteine proteinases papain, ficin, cathepsins B, H and L with similar inhibitory characteristics as recombinant cystatin-α. Proteinases of other classes were not inhibited. The inhibitory activity of HSP was heat stable in the wide pH range of 3.0–10.0. Polyclonal antibodies against HSP cross-reacted with cystatin-α and the molecular mass of HSP was similar to that of cystatin-α, though its isoelectric point was different. The in vivo location of both HSP and cystatin-α is on keratohyalin granules in epidermis as detected by indirect immunofluorescence technique using individual antibodies. Therefore it is highly probable that HSP is a cystatin-α derivative or a very similar proteinase inhibitor belonging to a family of cystatins.
