Calcium from isolated loops

Silicon tetrafluoride test This test depends upon the fact that when silica (isolated from a silicate by treatment with ammonium chloride solution or with hydrochloric acid, etc.) is heated with less than an equivalent amount of calcium fluoride and some concentrated sulphuric acid, silicon tetrafluoride is evolved. The latter is identified by its action upon a drop of water held in a loop of platinum wire, when a turbidity (due to silica) is produced. 

Conotoxins with the co-type 6-Cys/4-loop framework (C—C—CC—C—-C) are the most abundant group of peptides isolated from Conus venoms so far. This structural class encompasses at least four known pharmacological classes co-conotoxins that block voltage-sensitive calcium channels, 5-conotoxins that retard sodium channel inactivation, the sodium channel blocker conotoxin-GS, and two peptides recently found in C. marmoreus that affect both sodium and calcium currents. In addition, three co-type 6-Cys/4-Ioop peptides with still unknown targets have been found in C. textile (Cruz et al., 1992) and C. quercinus (Abogadie et al., 1990 Olivera et al., 1990). 

Conotoxin-GS, a sodium channel blocker isolated from C. geographus, has the 6-Cys/4-loop framework (C—C—CC—-C—C) of co-conotoxins rather than the 6-Cys/3-loop arrangement (CC—C—C—CC) of the p-conotoxins, the major Na channel blocker from C. geographus (Yanagawa et al., 1988). Conotoxin-GS also has the conserved Gly residue in the first loop typical of co-conotoxins. In fact, Yanagawa and co-workers have pointed out the greater sequence similarity of conotoxin-GS to the calcium channel blocker co-conotoxin MVIIA than to the p-conotoxin GUI series. Although the affinity of conotoxin-GS to sodium channels is much less than that of p-conotoxins, conotoxin-GS was also found to preferentially bind to Site I of the muscle rather than the neuronal subtype of sodium channels. 

Recently, two new peptides, MrVIA and MrVIB, were isolated from C. marmoreus venom (Fainzilber et al., 1995 McIntosh et al., 1995). These conotoxins were reported to affect both sodium and calcium currents in the freshwater snail Lymnaea stagnalis, although the toxins block Na currents more potently than calcium currents (Fainzilber et al., 1995). Of the groups of peptides with the 6-Cys/4-loop framework, MrVIA and MrVIB are the only ones that do not have the conserved Gly in the first intercysteine loop. The peptides have an unusual concentration of basic residues in the first loop with mainly hydrophobic amino acid residues in the other loops (Fainzilber et al., 1995), a feature mentioned above for the 5-conotoxins. 

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