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PROCESSES in BIOLOGICAL VISION: Including PROCESSES IN BIOLOGICAL VISION: including, ELECTROCHEMISTRY OF THE NEURON This material is excerpted from the full β-version of the text. The final printed version will be more concise due to further editing and economical constraints. A Table of Contents and an index are located at the end of this paper. James T. Fulton Vision Concepts [email protected] April 30, 2017 Copyright 2004 James T. Fulton Dynamics of Vision 7- 1 7 Dynamics of Vision 1 The dynamics of the visual process have not been assembled and presented in a cogent manner within the academic vision literature. On the other hand, several authors have presented cogent descriptions applicable to the clinical level. The material assembled by Salmon at Northeastern State University2 in Oklahoma is exemplary (but superficial for the purpose at hand). The dynamics associated with the mechanism of interpreting symbols and character groups, called reading, has not been presented at all. Only the major eye movements related to reading have been studied in significant detail. Even the adaptation characteristic of vision as a function of illumination level has not been presented from a theoretical perspective, and the empirical data has not been analyzed in sufficient detail to provide a coherent understanding of the process. When assembled as a group, the mechanisms and processes associated with forming the chromophores of vision provide a new, interesting and unique perspective on the formation of those chromophores. This Chapter will assemble the pertinent data with respect to a variety of processes where their dynamic aspects are crucial to the visual function. 7.1 Characteristics & Dynamics of Retinoids in the body The following material is based on extensive empirical investigations that were largely lacking with regard to any contiguous theory of what the goals of the mechanisms involved were from the perspective of the visual modality. The interpretations provided here take advantage of the hypothesis of this work; that the family of retinines, known as the Rhodonines() are the fundamental chromophores of animal vision. As noted in Section 3.6, these chromophores do not require chemical combination with an opsin to form the operational receptor of light. No opsin is found associated with the chromophores of Insecta or Mollusca. Opsin is a structural substrate in Chordata supporting the physical orientation of the chromophore coating the individual opsin discs. 7.1.1 Introduction This section will develop the dynamics of the retinoids from the perspective of transport through the blood stream and operation within the retina. Sections 4.6.2 & 4.6.3 developed the schematic aspects of the transport mechanisms related to both the transport and operation of the retinoids. A later chapter will develop the temporal characteristics associated with these processes as they relate to perception in vision. The complete absorption, transport and metabolism of the retinoids in the body are well beyond the scope of this work. However, a brief, first level, scenario is required to avoid the “floating model” trap and to interpret the known data concerning the retinoids in vision properly. The goal of this chapter is to present one such overall model and scenario. 7.1.1.1 Overall Baseline Vitamin A plays a major role in the animal body. The term retinol (the alcohol) and vitamin A are frequently considered synonymous in the clinical and biological literature. However, at a more detailed level, the term Vitamin A should probably be considered a synonym for retinoid that may be present in the alcohol, aldehyde or acid form. This modification allows these three forms to target different elements within the body and/or provides greater selectivity among the elements of the body for acquiring the desired chemical form. In 1994, Blomhoff edited an important volume on Vitamin A in Health & Disease3. On page 2 of this very important work, he cited the 1982 IUPAC–IUB Joint Commission on Biochemical Nomenclature, “Vitamin A is a generic term reserved to designate any compound possessing the biological activity of retinol, whereas the term retinoids includes both naturally occurring forms of vitamin A and the many synthetic analogs of retinol, with or without biological activity .” In this work, the term retinoids will be subdivided into two pertinent groups, the retinenes (non-resonant retinoids) and the retinines (resonant retinoids consisting of two oxygen atoms terminating a conjugated carbon chain). The retinines exhibit a spectral absorption that is non-isotropic and maximum along the axis connecting the two oxygen atoms. This resonant absorption is at a wavelength different from the isotropic absorption of the non resonant retinoids. 1Released: April 30, 2017 2Salmon, T. (2012) Vision Science IIIb: Binocular Vision http://arapaho.nsuok.edu/~salmonto/vs3.html 3Blomhoff, R. ed. (1994) Vitamin A in Health and Disease. NY: Marcel Dekker 2 Processes in Biological Vision The retinoids are normally transported from the liver to the target location by what are called retinoid binding proteins (RBP). These may be present in a variety of forms, both in the blood stream or individual cell types. The following discussions will only address the RBP’s related to the visual modality. These retinoids are absorbed from the intestine, aided by a series of enzymes that play a crucial role in preparing them for transport to the liver. The Blomhoff text is the most definitive work on this subject known at this time. Figure 1 of that text shows the stick versions of the molecules of interest here in their most recently agreed forms with the two methyl groups sharing a carbon of the β- ionone ring shown pointing upward. In actual fact, the retinoids are not planar molecules and cannot be adequately represented in a 2-dimensional configuration (Section 5.5.8). This configuration is used in the “2nd Order calculations (Section 5.5.8.3.1) of the an-isotropic spectral absorption of light by the retinine, Rhodonine(5) in this work. This absorption occurs at 610 nm under the endothermic conditions associated with most mammals. The numeric, 5, defines the location of the carbon bonding with the oxygen atom of the β-ionone ring in the L–channel chromophore of vision. 7.1.1.1.1 Scenario requirements Any realistic scenario must be anchored by the following factors: + The organism ingests either carotenes or Vitamin A as a source (the fundamental chromogens) of the chromophores of vision. + The blood stream of the animal is antagonistic to the delicate retinoids. The retinoids are particularly subject to oxidation. + Up to four separate types of Rhodonine (the actual chromophores) are deposited in liquid crystalline form on individual substrates of protein material in the Outer Segments of a mosaic of photoreceptor cells in each eye of the animal. The challenge is to determine at least one scenario in which the retinoids can traverse the available pathways between the small intestine and the retina without being exposed to destructive chemistry. 7.1.1.1.2 Reinterpretation of Data Base Until now, the mechanisms of the binding proteins related to vision have always been discussed on the assumption that they bound only the retinenes. The situation is more complex. The same generic complex, a retinol binding protein present in the serum (SRBP), and a molecule named transthyretin (TTR) may absorb/adsorb a retinol molecule and transport it to other locations unmodified, for a variety of purposes. It may also modify the retinol as part of the transport and/or delivery step. In the case of vision, there are clear indications that the delivered material is not a retinene but a retinine (with two i’s), a diol version of a retinene. The retinines may have been encountered in the biochemical laboratories studying SRBP and the CRBP/CRABP’s where they have been described as non-canonical forms of retinol. A definition of non-canonical form may be difficult to find. For the purpose of this discussion, “the basic concept behind the canonical structure is whether two consecutive bonds are appearing in equilibrium or not; C if the bonds appear to be in equilibrium the situation will be called canonical, C if not, the situation will be non-canonical and the process can be well understood by resonance.” Resonance is distinguished from isomerism. An isomer is a molecule with the same chemical formula but with different arrangements of atoms in space. Resonance contributors of a molecule, on the contrary, can only differ by the arrangements of electrons. Therefore the resonance hybrid cannot be represented by a combination of isomers4. Upon recognizing that both the retinenes and the retinines are processed by these binding proteins, readdressing the data base in the literature is necessary. This is necessary to discover how the additional process flexibility introduced by this situation is used and where the resultant processes take place. Much of the information associated with the SRBP + TTR complex has been gather in the context of its use in nutrition. However, the complex is used in many other contexts, including vision but also in transporting material to the testes in male animals as an example. 4http://en.wikipedia.org/wiki/Resonance_%28chemistry%29 Dynamics of Vision 7- 3 Furthermore, the literature is generally unclear whether the particular author is speaking about the holo- or apo- form of the protein. Making this distinction is absolutely necessary if the dynamics of the transport mechanism is to be understood. In the visual case, the post transport apo- protein may not be identical to the pre transport apo- form (see Section 7.1.1.2.3). Most of the stoichiometry of the SRBP + TTR complex has focused in the fact that a complex of two molecules of SRBP and one molecule of TTR has been found easiest to crystallize for purposes of X-ray analysis in order to gain maximum knowledge of the conformation of these materials in complex.
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