Properties and nature of a cysteine proteinase inhibitor located in keratohyalin granules of rat epidermis. 
The pI 4.7, 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-alpha.  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-alpha and the molecular mass of HSP was similar to that of cystatin-alpha, though its isoelectric point was different.  The in vivo location of both HSP and cystatin-alpha 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-alpha derivative or a very similar proteinase inhibitor belonging to a family of cystatins.