Nedelkov D, Bieber A L
Department of Chemistry and Biochemistry, Arizona State University, Tempe 85287-1604, USA.
Toxicon. 1997 May;35(5):689-98. doi: 10.1016/s0041-0101(96)00175-4.
The two myotoxin a isomers from the venom of the prairie rattlesnake Crotalus viridis viridis have different isoelectric points, as determined by capillary zone electrophoresis. The pI values are 10.50 and 10.57, respectively, and both are higher than the previously reported pI value 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 during the purification process. Fluorescence quenching of myotoxin a reveals heterogeneity of the tryptophans, possibly due to different environments. The fraction of the total tryptophan fluorescence quenched by iodide is 81% and is attributed to solvent-accessible tryptophan residues at the protein surface. The 19% non-quenchable tryptophans probably represent residues that are shielded from the solvent exposure. The ratio of buried to exposed tryptophans is similar to the ratio of isomers seen by capillary zone electrophoresis and reverse-phase high-performance liquid chromatography (c. 1 : 4).
通过毛细管区带电泳测定，来自草原响尾蛇（Crotalus viridis viridis）毒液的两种肌毒素a异构体具有不同的等电点。其pI值分别为10.50和10.57，两者均高于先前报道的肌毒素a的pI值。两种异构体等电点的差异归因于肌毒素a构象变化导致的表面电荷改变。两种异构体均存在于粗毒液中，排除了它们是在纯化过程中形成的假象的可能性。肌毒素a的荧光猝灭揭示了色氨酸的异质性，这可能是由于不同的环境所致。被碘化物猝灭的总色氨酸荧光比例为81%，这归因于蛋白质表面可被溶剂接触的色氨酸残基。19%不可猝灭的色氨酸可能代表被屏蔽而不暴露于溶剂的残基。埋藏色氨酸与暴露色氨酸的比例类似于通过毛细管区带电泳和反相高效液相色谱法观察到的异构体比例（约为1:4）。
