Retinal oxygenation

Wangsa-Wirawan ND, Linsenmeier RA (2003) Retinal oxygen Fundamental and clinical aspects. Arch Ophthalmol 121 547-557. 

Because of the presence of an extended polyene chain, the chemical and physical properties of the retinoids and carotenoids are dominated by this feature. Vitamin A and related substances are yellow compounds which are unstable in the presence of oxygen and light. This decay can be accelerated by heat and trace metals. Retinol is stable to base but is subject to acid-cataly2ed dehydration in the presence of dilute acids to yield anhydrovitamin A [1224-18-8] (16). Retro-vitamin A [16729-22-9] (17) is obtained by treatment of retinol in the presence of concentrated hydrobromic acid. In the case of retinoic acid and retinal, reisomerization is possible after conversion to appropriate derivatives such as the acid chloride or the hydroquinone adduct. Table 1 Hsts the physical properties of -carotene [7235-40-7] and vitamin A. 

Plastoquinone, whose line structure appears in the margin, has ten double bonds. Unlike the bonds in the tail of retinal, however, the bonds in the long tail of plastoquinone are not delocalized because s p -hybridized carbon atoms separate them. The delocalized 7t system of plastoquinone is its planar ring of six carbon atoms, with two of the carbon atoms double-bonded to outer oxygen atoms. 

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

Flower, R.W. and Patz, A. (1971). Oxygen studies in retrolental fibroplasia. IX. The effects of elevated arterial oxygen tension on retinal vascular dynamics in the kitten. Arch. Ophthamol. 85, 197-208. 

Rao, N.A. (1990). Role of oxygen free radicals in retinal damage associated with experimental uveitis. Trans. Am. Ophthalmol. Soc. 88, 797-850. 

Moreover, carotenoids may quench electronically excited states and scavenge free radicals formed in the retina, and therefore protect biomolecules from oxidative damage. Due to the low energy level of the first excited triplet state ( Car), carotenoids (Car) can act as efficient acceptors of triplet state energy from photosensitizers (S) (Equation 15.1), such as all-tra .s-retinal, the photosensitizers of lipofuscin (Rozanowska et al., 1998), or singlet oxygen C02) (Equation 15.2) (Cantrell et al., 2003) ... 

Rozanowska, M, Jarvis-Evans, J, Korytowski, W, Boulton, ME, Burke, JM, and Sarna, T, 1995. Blue light-induced reactivity of retinal age pigment. In vitro generation of oxygen-reactive species. J Biol Chem 270, 18825-18830. 

Rozanowska, M, Wessels, J, Boulton, M, Burke, JM, Rodgers, MA, Truscott, TG, and Sarna, T, 1998. Blue light-induced singlet oxygen generation by retinal lipofuscin in non-polar media. Free Radio Biol Med 24, 1107-1112. 

Examples of silicones in ophthalmic application have been described.5 1 However, some problems have been reported with the use of silicones in retinal tamponade.542 Silicones are widely used in contact lenses because of their unusually good oxygen permeability and bioinertness, but the need for novel materials continues. Low-modulus fluorosiloxane-based hydrogel lenses with a wide range of water content and high-oxygen permeability have been described in a number of papers.543-... 

In ophthalmological application, this characteristic of the PFCLs is not used yet. In general, the products used are air equilibrated with the consequence that the oxygen partial pressure in the eye is increased from 15 Torr to 160 Torr and the CO2 partial pressure drops down from 50 Torr to 3 Torr initially. These differences are equilibrated intra-ocularly by diffusion processes, but the initial difference to the physiological level of gas concentration activates the constriction of the retinal vessels, resulting in an increase of the blood flow. In rabbit eyes, a damage of the retina could be attributed to this mechanism [38,39], On the other hand, endotamponade media with controlled levels of dissolved gases could not only avoid such a scenario but should also be useable for a therapeutic manipulation of the retinal perfusion. 

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