Retinoid numbering systems

Detection can be performed using UV, UV-Vis, DAD, fluorometric detector (ELD) or ED. Gatti et al. [444] report an on-line photochemical derivatization to enhance the selectivity. A photo reactor system was inserted on-line between the column and the ELD. The eluate was irradiated at 254 and 366 nm inducing alterations that modified spectral properties of the analytes. Recently an increasing number of studies have reported the use of LC-MS for the identification of both retinoids and carotenoids [426,433,445-447],... 

This Chapter will present the actual chromophores of vision, labeled the Rhodonines and derivable from a number of feedstocks, including the retinol family, consist of relatively small molecules with a molecular weight of either 285 (R5 R9) or 299 (R7 R11). They are retinoids of the resonant conjugate type. They are also carboxylic-ion systems and exhibit a negative charge in their fundamental form. The molecules are relatively easily generated in the laboratory in pure form. However, they exhibit a number of unique properties that have made their isolation difficult. They only exhibit the properties of a visual chromophore when in the liquid crystalline state. Their absorption characteristic is a transient one unless a means of de-exciting the molecules of the liquid crystal is present. Finally, they are extremely sensitive to destruction by oxidants and alkali metal ions. 

The Rhodonines, as defined in this work, are the actual chromophores of vision when in the liquid crystalline state and supported by an auxiliary de-excitation mechanism. They can be placed in the chemical hierarchy in a number of ways. Some of these are addressed by Zollinger207 from the perspective of a dye. Others are addressed by Kennard, et. al from a crystallographers perspective208. At the current time, it is most appropriate to define the Rhodonines in terms of the broad retinoid family. This family also includes the retinenes which the Rhodonines resemble. The retinenes are chromogens of the Rhodonines. It is the Rhodonines which are the actual chromophores of vision. Within this family, the unique characteristic of the Rhodonines is their carboxylic-ion system. 

A number of other nuclear receptors have also been shown to bind and be activated by retinoids. For example, testicular receptor 4 (TR4) has recently been suggested to be a retinoid receptor, because the binding of both retinol and RA can induce conformational changes of TR4 leading to the activation of this receptor (Zhou et al. 2011). TR4 can either function as homodimer or heterodimerize with testicular receptor 2 (TR2) to function in spermatogenesis, hpid and hpoprotein regulation, and central nervous system development (Zhou et al. 2011). However, additional experiments are required to reveal the physiological and developmental implications of the activation of TR4 by retinoids, such as RA (Zhou et al. 2011). 

A large number of retinoids in which the cyclogeranyl ring of the natural retinoids has been replaced by a different ring system have been prepared using the reaction sequences previously developed for the synthesis of retinol (1),... 

For further work, serum has been replaced by insulin and transferrin, and a routine assay for retinoids in serum-free medium is performed as follows (Breitman et al., 1980a) HL-60 cells are seeded at a density of 2 x 10 cells per ml in defined medium, which is a 1 1 mixture of Dulbecco s modified Eagle s minimum essential medium and Ham s F-12 medium supplemented with 3 x 10 M selenium dioxide, insulin (5 xg per ml), and transferrin (5 ig per ml). Retinoids are added to the HL-60 cells at the start of the assay, using ethanol as the vehicle final concentration of ethanol does not exceed 0.1%. The cells are incubated for 4 or 5 days, and differentiation is then measured by NBT reduction (Collins et al., 1979) of cytospin slide preparations. Results are expressed as percentage of NBT positive cells. Approximately 4-8% of the cells will differentiate spontaneously in the absence of added retinoid. As with the F9 system, dose-response curves have been determined for a large number of retinoids, and some of the more significant results are shown in Table III. 

Although results obtained in this test system are of major importance for understanding of cellular and molecular mechanisms (these are discussed at length in Chapter 12), this is not a practical routine assay for evaluating large numbers of retinoids. Personnel highly experienced in cell culture are required for proper maintenance of the cell line, and the assay takes 5-6 weeks to complete. 

To name retinoids systematically (lUPAC nomenclature), however, a different numbering scheme must be used the carbon atom bonded to the functional group is given number 1. The numbering of carbon atoms by this scheme is shown m structure III for a -trans retinoic acid. Accordingly, the systemic name for a -trans retinoic acid is (al trans) 3,7-dimethyl-9-(2,6,6-trimethylcyclohex-l-en-l-yl)-nona-2,4,6,8-tetraen-l-oic acid, or more simply (all-trans) 3,7-dimethyl-9-(2,6,6-trimethylcyclohexene-l-yl)-2,4,6,8-nonatetraenoic acid. 

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