CHARACTERIZATION OF THE 2 MYOTOXIN-A ISOMERS FROM THE PRAIRIE RATTLESNAKE (CROTALUS-VIRIDIS-VIRIDIS) BY CAPILLARY ZONE ELECTROPHORESIS AND FLUORESCENCE QUENCHING STUDIES

The two myotoxin a isomers from the venom of the prairie rattlesnake C rotalus viridis viridis have different isoelectric points, as determin ed by capillary zone electrophoresis. The pI values are 10.50 and 10.5 7, respectively, and both are higher than the previously reported pI v alue for myotoxin a. The difference in the isoelectric points between the two isomers is attributed to altered surface charge as a result of the conformational change in myotoxin a. Both isomers exist in crude venom, discounting the possibility that they are artifacts formed duri ng the purification process. Fluorescence quenching of myotoxin a reve als heterogeneity of the tryptophans, possibly due to different enviro nments. The fraction of the total tryptophan fluorescence quenched by iodide is 81% and is attributed to solvent-accessible tryptophan resid ues at the protein surface. The 19% non-quenchable tryptophans probabl y represent residues that are shielded from the solvent exposure. The ratio of buried to exposed tryptophans is similar to the ratio of isom ers seen by capillary zone electrophoresis and reverse-phase high-perf ormance liquid chromatography (c. 1:4). Copyright (C) 1997 Elsevier Sc ience Ltd.