Primary structure of the antihemorrhagic factor in serum of the Japanese Habu: a snake venom metalloproteinase inhibitor with a double-headed cystatin domain.

The complete amino acid sequence of an antihemorrhagic factor, HSF, in the serum of the Japanese Habu snake, Trimeresurus flavoviridis, has been determined. The protein is composed of 323 amino acid residues and contains three asparagine-linked oligosaccharide chains at positions 123, 185, and 263. The molecule contains two copies of the cystatin domain in the N-terminal portion up to position 240, and these domains show a remarkable sequence homology (about 50%) to those of plasma glycoproteins such as alpha 2-HS (human) and fetuin (bovine) and to a lesser extent to that of HRG (human). The amino acid sequence of the noncystatin region towards the C-terminus is unique, showing no significant homology with those of the corresponding regions of alpha 2-HS and fetuin. In spite of the presence of cystatin domains, HSF does not inhibit cysteine proteinases such as papain and cathepsin B but does inhibit several metalloproteases in Habu venom. The results suggest that HSF is the first protein found to be functionally related to metalloproteinase inhibitors among the structurally homologous proteins with a double-headed cystatin domain, and is a member of a novel family (family 4) with divergent functions of the cystatin superfamily proteinase inhibitors. Although HSF possesses similar physicochemical properties to those of oprin, a snake venom metalloproteinase inhibitor with antihemorrhagic activity isolated from opossum serum [Catanese & Kress (1992) Biochemistry 31, 410-418], its primary structure is strikingly different from that of oprin.