Myotoxic Phospholipases A2 from Venoms of Crotalid Snakes: Properties, Actions, and Neutralization
Lomonte, Bruno
Instituto Clodomiro Picado, Facultad de Microbiología, Universidad de Costa Rica, San José, Costa Rica.
Snakes from the family Crotalidae, especially Bothrops spp. account for most envenomings in Latin America. Their venoms have abundant phospholipases A2 (PLA2S), including those involved in local damage to skeletal muscle (myotoxic PLA2S or myotoxins). The isolation of myotoxins from Bothrops and Agkistrodon venoms began in the 1980's, adding to the previously known crotoxin from Crotalus durissus terrificus (which causes systemic myonecrosis). Biochemical characterization has shown that these toxins have a class 11 PLA2 structure, molecular weights of 13 to 16 kDa, 121 to 123 amino acids, and basic isoelectric points. Although some of them appear as monomers, several are found as non-covalently associated homodimers. Sequence analyses have revealed a separation of the myotoxic class 11 PLA2S into two groups: "Asp49 " and "Lys49". The first proteins have the key amino acids for catalyzing phospholipid hydrolysis. However, the Lys49 myotoxins display critical amino acid substitutions in their sequences (including the Asp49/Lys49 change) that preclude the catalytic action. Since early hypotheses on the mechanism of muscle damage were based on the ability of myotoxic PLA2S to hydrolyze cell membrane phospholipids, the discovery of myotoxic PLA2-like proteins, unable to perform enzymatic catalysis, implied a reassessment of their mode of action. Both types, the Asp49 and the Lys49 myotoxins, share the ability to induce rapid local myonecrosis and edema in vivo. In vitro, both can lyse a variety of cell types (especially skeletal muscle cells), artificial phospholipid vesicles, and even bacteria.
In recent years, significant progress in the structural characterization of myotoxic PLA2s has been achieved, with a growing number of known sequences and 3-D structures. Also, important evidence for the identification of the molecular region(s) involved in their toxic actions has been gathered. At least in the Lys49 group, a cationic/hydrophobic region close to the C-terminal end (1 15-129) is known to account for direct damage to biomembranes, including the myotoxic effect. However, important aspects on the mode of action of myotoxins are still unknown. A fundamental issue is that to the "receptor" or "acceptor" site for these toxins are so far unidentified.
Neutralization studies on the myotoxic PLA2s demonstrated that they constitute the most relevant toxins in the pathogenesis of muscle damage caused by crotalid venoms. The list of neutralizing agents studied in the recent years includes polyclonal and monoclonal antibodies, heparins, plant extracts, chemical agents, and protein inhibitors present in snake plasma. In general, results have shown that an efficient neutralization of myotoxins in vivo is a difficult task, even with agents that display excellent neutralizing properties in assays in vitro, and this has precluded novel clinical applications so far. However, the growing basic information accumulating on these toxins could soon pave the way to the introduction of substantial therapeutic improvements. Immunoprophylaxis to myotoxins is another alternative just now being explored in search of a solution to the problem of myonecrosis caused by crotalid snake venoms.
Publication Dates
-
Publication in this collection
08 Oct 2002 -
Date of issue
Dec 2001
