Visual pigments, bleaching

Two hypotheses have been forwarded to account for this blocking of the Na + channels ([1] for a recent review, see [2]). The first is that Ca2+ ions, which are released from the inside of the discs into the cytoplasmic space upon illumination, are responsible for the shut off of the dark current. The second hypothesis casts cyclic GMP into the role of the transmitter upon absorption of light by a visual pigment, activation of a phosphodiesterase occurs which reduces the levels of cyclic GMP (5 X 104 molecules of cyclic GMP disappear upon bleaching of one molecule of pigment). This in turn results in a reduced activity of a protein kinase which is... 

Photoisomerization.—Birge and Hubbard analyse the molecular dynamics of cis-trans isomerization in the visual pigment rhodopsin using INDO-CISD molecular orbital theory and semiempirical molecular dynamic theory. The analysis predicts that the excited-state species is trapped during isomerization in an activated complex that has a lifetime of 0.5ps. This activated species oscillates between two components which preferentially decay to form isomerized product (bathorhodopsin) or unisomerized 11-cw-chromophore (rhodopsin) within 1.9—2.3ps. The authors further conclude that the chromophore in bathorhodopsin has a distorted all-rraw-geometry and is the most realistic model for the first intermediate in the bleaching cycle of rhodopsin. 

We have seen that the rods and cones contain the visual pigment and that the pigment bleaches when exposed to light. The retina of the intact animal recovers its bright color when placed in the dark. Such... 

Fig. 4.18 The rhodopsin cycle. The bleaching of rhodopsin yields all-trans retinene (now more currently indicated as retinal), which must be isomerized to neoretinene b before it can regenerate the visual pigment. Alternatively, having been reduced to all trans vitamin A, the latter in turn must be isomerized to neovitamin Ab before it takes part in rhodopsin synthesis. Rod vision thus depends on the continuous stereoisomerization of all-trans retinene or vitamin A, or on the continuous replacement of these substances by new supplies of vitamin Ab from external sources (adapted from [196, 197])...

In addition to entering the RPE from the circulation, all-lrany-retinol passes into the RPE cells through their apical surfaces during bleaching of ihodopsin in the ROS. This phase of the visual cycle is better characterized than the reverse step, namely the reverse flow of retinoid that culminates in visual pigment regeneration. 

The visual pigments are strongly dichroic, showing that their chromophores are oriented within the membrane. When bleaching occurs in isolated rod outer segments, the retinol that accumulates also exhibits a high dichroic ratio but in the opposite sense (Harosi and MacNichol, 1974). 

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