Isolation and identification of a snake venom metalloproteinase inhibitor from California ground squirrel (Spermophilus beecheyi) blood sera.

California ground squirrels (Spermophilus beecheyi) show blood-based defenses to a variety of toxins in the venom of the Northern Pacific rattlesnake (Crotalus oreganus oreganus). In this study we demonstrate the presence of an effective snake venom metalloproteinase inhibitor (SVMPI) in S. beecheyi. The blood sera of California ground squirrels were effective at reducing the metalloproteinase activity of Northern Pacific (C. o. oreganus) and prairie rattlesnake (Crotalus viridis viridis) venoms by over 75%, significantly more than its ability to reduce the activity of western diamondback rattlesnake venom. We used anion exchange and affinity chromatography to isolate this protein from the blood sera of S. beecheyi. This SVMPI had a molecular mass of 108.3 kDa and a pI of 5.1. The IC(50) of this inhibitor against whole venom from C. o. oreganus was determined to be 3.14 × 10(-8) M. Subsequent LC MS/MS analysis of a CNBr/tryptic digest of the inhibitor yielded multiple internal peptide sequences. These sequences showed homology to three other known mammalian plasma proteins: inter-α trypsin inhibitor, and two hibernation-associated proteins, HP25 and HP27. The presence of SVMPI in S. beecheyi blood sera is consistent with the resistance of these animals to venom-induced hemorrhage and tissue damage, and consistent with the protective factors conferring venom resistance in other mammals. However, the variety of SVMPI identified to date from mammalian taxa suggests that different species have converged on neutralization of venom metalloproteinase activity as a key step in venom neutralization.