Purified cobramine B from Naja naja inhibits the accumulation of I- by thyroid and parotid slices or isolated choroid plexus. Accumulation of TcO4- and ReO4- is also inhibited. The effect has a rapid onset, is temperature-dependent, and is prevented by polyanions or antivenom, but it is reversed with great difficulty, presumably because of very firm binding to tissue. Pretreatment with neuraminidase does not influence the inhibition. The major portion of the inhibition is due to increased I- efflux from the slices. Significant increases occur at 2 µg per ml, and I- efflux is a more sensitive measure of the cobramine B effect than is I- accumulation. Cobramine B leads to K+ loss from the cell and increases the rate of equilibration of the sucrose or inulin spaces. Inhibition of I- accumulation is also produced by protamine, histones, polyarginine, polyornithine, and polylysine, but not by lysozyme or ribonuclease A. Tetralysine is ineffective, and a polylysine of average molecular weight 14,000 or greater is maximally effective. Larger polymers are not more potent. As with cobramine B, inhibition is prevented but not easily reversed by polyanions. It is postulated that cobramine B interacts with the negative surface charges of the thyroid cell.
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Wolff et al. (1968) studied this question.