Autologous extracellular cytochrome c is an endogenous ligand for leucine-rich α2-glycoprotein and β-type phospholipase A2 inhibitor

Abstract

β-Type phospholipase A2 inhibitory protein (PLIβ) from the serum of the venomous snake Gloydius brevicaudus neutralizes basic phospholipase A2 (PLA2) from its own venom, and it has 33% sequence homology with human leucine-rich α2-glycoprotein (LRG), which has been recently reported to bind cytochrome c (Cyt c) (Cummings, C., Walder, J., Treeful, A., and Jemmerson, R. (2006) Apoptosis 11, 1121-1129). In the present study, PLIβ was found to bind Cyt c. The interactions ofLRGand PLIβ with Cyt c were compared by surface plasmon resonance analysis. Human LRG bound horse and snake Cyt c with dissociation constants of 1.58 x 10-13M and 1.65 x 10-10M, respectively, but did not bind yeast Cyt c, while G. brevicaudus PLIβ bound horse, snake, and yeast Cyt c with dissociation constants of 1.05 x 10-10 M, 2.37 x 10 -12 M, and 1.67 x 10-6 M, respectively. On the other hand, LRG did not show any PLA2 inhibitory activity and did not bind G. brevicaudus basic PLA2, whereas PLIβ bound the basic PLA 2 with a dissociation constant of 1.21 x 10-9 M, which is smaller than those with the Cyt c described above. The PLA2 inhibitory activity of PLIβ was also found to be suppressed by the binding of Cyt c to PLIβ. These results suggest that autologous Cyt c is an endogeneous ligand for LRG and PLIβ and that these serum proteins neutralize the autologous Cyt c released from the dead cells. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.