Target
Ophanin is a weak blocker of the high potassium-induced contraction of smooth muscles. Snake venom CRISP family proteins inhibit depolarization-induced smooth muscle contraction to different extents. Compared to the normal contraction of smooth muscle, ophanin is able to reduce their force of contractility to 84% ± 1%, which is less than most other CRISPs.
The differences between the inhibitory activity of CRISPs may be explained through sequence comparisons that suggest a site that may be critical for inhibition of channel activity. Phe189 and Glu186 are the most likely functional residues: strong blockers of smooth muscle contraction (ablomin, triflin, and latisemin) all have Phe189, and all blockers of smooth muscle contraction except ophanin have Glu186. The significance of this lack of the probable functional residues in ophanin has not yet been addressed. However, it is likely that the picture is more complex and other residues contribute to the inhibitory activity of CRISPs on smooth muscle contraction and some data supports this. For example pseudecin, while also having Phe189, does not affect depolarization-induced contraction.
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