Thallium-201: autoradiography in pigmented mice and melanin-binding in vitro

Autoradiography with 201Tl+ in C57Bl mice showed a strong labelling of the eye melanin and of pigmented hair follicles. An analysis of the affinity of thallium for pigment from cow eyes indicated a binding to three groups of sites and showed a marked sensitivity to the addition of H+-ions. The results are consistent with the conception that a binding of thallium occurs to the free carboxyl groups of the melanin and that the structure of the polymer has a marked influence on the affinity. Similar results have previously been obtained with other cations. There was no indication that the strong in vivo affinity of thallium to melanin is due to a more firm binding than for other cations which do not localize on melanin in vivo. Instead, the ability of cations to pass the melanocyte membranes and reach the melanin granules is probably decisive to whether a melanin binding will take place in vivo. Toxic effects on the eye and epilation are symptoms of thallium intoxication which may be related to its melanin binding. The fate of 201Tl+ in some other tissues is also described and discussed.