Multiple binding sites for melatonin on Kv1.3

Melatonin is a small amino acid derivative hormone of the pineal gland. Mel atonin quickly and reversibly blocked Kv1.3 channels, the predominant volta ge-gated potassium channel in human T-lymphocytes, acting from the extracel lular side. The block did not show state or voltage dependence and was asso ciated with an increased inactivation rate of the current. A half-blocking concentration of 1.5 mM was obtained from the reduction of the peak current . We explored several models to describe the stoichiometry of melatonin-Kv1 .3 interaction considering one or four independent binding sites per channe l. The model in which the occupancy of one of four binding sites by melaton in is sufficient to block the channels gives the best fit to the dose-respo nse relationship, although all four binding sites can be occupied by the dr ug. The dissociation constant for the individual binding sites is 8.11 mM, Parallel application of charybdotoxin and melatonin showed that both:compou nds can simultaneously bind to the channels, thereby localizing the melaton in binding site out of the pore region. However, binding of tetraethylammon ium to its receptor decreases the melatonin affinity, and vice versa. Thus, the occupancy of the two separate receptor sites allosterically modulates each other.