The gecko visual pigments—I. The thermosensitive property

Abstract

The photosensitive retinal pigments of geckos extracted into aqueous digitonin solutions are characterized by an especially high sensitivity to temperature. Unlike frog rhodopsin, these pigments are bleached when the temperature is raised from 5°C. Even an increase of 5–10°C, produces some bleaching. The effect of a step increase in temperature is to bleach away the pigment to some equilibrium level depending on the magnitude of the temperature increase. Restoring the temperature back to 5°C results in a rapid regeneration of photopigment, often completely reversible. The loss and regeneration of pigment density follows closely the change in temperature, being, in effect, a thermometric property. This behavior of the gecko pigments follows temperatures up to about 30°C. Above 30°C other effects apparently occur which remain to be studied in detail. The action of these mild temperatures has been hypothesized to be an attack on the opsin, changing its conformation in an easily reversible manner. No isomerization of the 11- cis retinal is conceived to occur. This hypothesis supports the idea of an effect of mild heat quite different from light and also different from the action of higher temperatures. The former is considered to lead to isomerization as a primary process while the latter is usually viewed as resulting in a denaturation of the visual protein. The unique thermal lability of the gecko photopigments suggests that these substances may be of special value in the study of certain chemical and biological problems in visual science.