Subunit structure and inhibition specificity of α-type phospholipase A2 inhibitor from Protobothrops flavoviridis

The α-type phospholipase A2 inhibitor (PLIα) in the plasma of the Habu snake, Protobothrop flavoviridis, was shown to be a trimer of two homologous subunits, PLIα-A and PLIα-B, each of which contains one C-type lectin-like domain (CTLD). Since one molecule of trimeric PLIα binds stoichiometrically to one molecule of P. flavoviridis acidic phospholipase A2 (PLA2), the trimeric structure is critical for its inhibitory activity. Hydrophobic chromatography separated the purified P. flavoviridis PLIα into four different trimeric subspecies, A3-PLIα, A2B-PLIα, AB2-PLIα, and B3-PLIα, with different combinations of the two subunits. The trimeric PLIα could be reconstituted from the purified subunits, and the four different trimeric subspecies were formed through random association of the two subunits. The inhibitory activity of the PLIα-A homotrimer (A3-PLIα) was more specific than that of the PLIα-B homotrimer (B3-PLIα). This difference in inhibitory properties between the two homotrimers was probably caused by the amino acid differences at residues 10–37.