Cystatin domains in alpha-2-HS-glycoprotein and fetuin.

We have found that chain A of alpha-2-HS-glycoprotein contains two cystatin domains that show closest similarity to those of kininogen. Most likely, the two proteins diverged after the primary duplication of a single cystatin domain as the two cystatin domains of alpha-2-HS-glycoprotein are more similar, especially in disulfide bonding, to the corresponding domains of kininogen than to each other. We also propose that the carboxyl-terminal (non-cystatin) parts of kininogen and alpha-2-HS-glycoprotein contain homologous segments. We suggest that alpha-2-HS-glycoprotein may act as an inhibitor of the cysteine proteinases responsible for bone resorption. We have also found that fetuin is closely related to alpha-2-HS-glycoprotein.