Autologous Extracellular Cytochrome c Is an Endogenous Ligand for Leucine-rich α2-Glycoprotein and β-Type Phospholipase A2 Inhibitor

beta-Type phospholipase A(2) inhibitory protein (PLI beta) from the serum of the venomous snake Gloydius brevicaudus neutralizes basic phospholipase A(2) (PLA(2)) from its own venom, and it has 33% sequence homology with human leucine-rich alpha(2)-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, PLI beta was found to bind Cyt c. The interactions of LRG and PLI beta 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(-13) M and 1.65 x 10(-10) M, respectively, but did not bind yeast Cyt c, while G. brevicaudus PLI beta 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 PLA(2) inhibitory activity and did not bind G. brevicaudus basic PLA(2), whereas PLI beta 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 PLA(2) inhibitory activity of PLI beta was also found to be suppressed by the binding of Cyt c to PLI beta. These results suggest that autologous Cyt c is an endogeneous ligand for LRG and PLI beta and that these serum proteins neutralize the autologous Cyt c released from the dead cells.