Retinoyl-p-glucuronide

Besides synthetic derivatives useful in therapy there exists a natural derivative of retinol (98), and retinoic acid(s) e. g., all-fraws-retinyl 8-glucuronide (99) [183] and all-fraws-retinoyl P-glucuronide (100) [184]. These vitamin A glycosides were first identified as billiary mefabo-lites of vitamin A, and they are now prepared also chemically for their favorable biological effects [185,186]. 

Retinoic acid from the intestinal mucosa is transported bound to serum albumin via the portal vein. Retinoic acid cannot be significantly reduced to retinal but is rapidly metabolized in tissue, such as Uver, to yield more polar catabohtes (e.g., 5,6-epoxyretinoic acid) and conjugates, such as retinoyl p-glucuronide, that are excreted. A small amount of retinoic acid undergoes enterohepatic circulation after intestinal hydrolysis of the glucuronide is excreted in the bile. 

Another metabolite, which may be very important, is retinoyl p-glucuronide.P... 

Kraft, J. C., Bechter, R., Lee, Q. R, and Juchau, M. R. (1992) Microinjections of cultured rat conceptuses Studies with 4-oxo-all-trans-retinoic acid, 4-oxo-13-ds-retinoic acid and all-tra i-retinoyl-P-glucuronide. Teratology 45,259-270. Maden, M. (1983) The effect of vitamin A on the regenerating axolotl limb. 

When (all- )-retinoic acid (3) was administered orally to rats, it was observed that (all- )-retinoyl-p-glucuronide (207) was rapidly excreted in the bile (Dun-agin et al., 1965 Lippel and Olson, 1968 Zile et al., 1980). The compound... 

In the plasma, vitamin A is transported from its major depots in the liver to tissues in several forms, primarily as a 1 1 molar complex of all-frans-retinol with RBP. Low concentrations of dll-trans- and 13-ds-retinoic acid, probably bound to albumin, and retinyl and retinoyl p-glucuronides, are also present. Holo-RBP also interacts strongly with transthyretin in the plasma. All of these forms of vitamin A can be taken up by various tissue cells. Several tissues besides the parenchymal cells of the liver can also synthesize RBP, as evidenced by the presence of mRNA for RBP within such cells. Thus, the extensive recycling of retinol between the liver and peripheral tissue cells may well occur as complexes with RBP. Another possibility is that retinyl ester, which is synthesized in essentially all cells of the body, might be carried back to the liver in lipoproteins. 

The current leading hypothesis is that nuclear RAR and RXR play a direct role in this process. The retinoid receptors can be activated by physicochemical binding of free retinoic acid to RAR and RXR. Alternatively, covalent forms, such as retinoyl derivatives of RAR and RXR, might also exist. Interestingly, retinyl and retinoyl p-glucuronide stimulate the differentiations of HL-60 cells well without evident conversion to retinol and retinoic acid, respectively. Retinoic acid has also been implicated as a morphogen in embryonic development (18). The adverse effects of vitamin A deficiency on reproduction, growth, and the immune response, in all likelihood, are an expression of perturbations in the process of cellular differentiation. 

Meyer et al. analyzed plasma retinol plus endogenous aW-trans and i-cis retinoic acid isocratically by normal-phase HPLC, using hexane 2-propanol acetic acid as mobile phase (90). Extraction of the retinoic acids required acidification of the sample however, too much acid can result in dehydration of retinol to anhydroretinol, and hydrolysis of endogenous retinoyl P-glucuronide (90). A synthetic retinoid sulfonic acid was used as internal standard. By using absorbance at 350 nm, limits of detection were 1.7 nM (0.5 (ig/L) for retinoic acid isomers, 35 nM (10 Xg/L) for retinol. 13-Demethyl retinoic acid has also been used as internal standard (142). Lanvers et al. used normal-phase HPLC with gradient elution (hexane 2-propanol glacial acetic acid) to analyze 13-c/x retinoic acid, 9-cis retinoic acid, aW-trans retinoic acid, retinol, and the 4-oxo metabolites of the retinoic acid isomers (143). Plasma samples were treated with ethanol to denature proteins, and then were extracted with hexane after addition of saturated ammonium sulfate solution (pH 5). 

Figure 4 Reversed-phase HPLC elution profiles of tocopherols (panel A), retinoids (B), and carotenoids (C) present in human plasma (200 pL). Blood was collected 3 h after an oral dose of retinoic acid. The chromatogram was obtained by use of gradient elution (Table 3). Peak identification 2, 4-oxo-retinoic acid 4, retinoyl P-glucuronide 7, retinoic acid 8, retinol 9, retinyl acetate (internal standard) 15, butylated hydroxy toluene 16, y-tocopherol 17, a-tocopherol 18, free bilirubin 19, lutein 20, zeaxanthin 21, 2, 3 -anhydrolutein 22, P-cryptoxanthin 23, lycopene 24, a-carotene 25, P-carotene. (From Ref. 73.)...

Electron impact ionization (El) and chemical ionization (Cl) mass spectrometry using a direct insertion probe continue to be used for molecular weight confirmation and identification of purified retinoids. Retinoid fragmentation patterns are useful for identification, especially when mass spectra of unknown compounds are compared to those of reference standards. For example. Buck et al. (284) used El and Cl mass spectrometry with a direct insertion probe to identify retinol as an essential growth factor for the culturing of human B cells, and Lakshman et al. (285,286) used El mass spectrometry to identify retinal O-ethyloxime. Barua (287) reported the desorption chemical ionization mass spectra of retinoyl P-glucuronide after methylation with diazomethane and pertrimethylsilylation. Molecular ions were detected in very low abundance. 

Bama AB (1997) Retinoyl p-glucuronide a biologically active form of vitamin A. Nutr Rev 55 259-267... 

The parent compound in the vitamin A group is called sdl-trans retinol (Fig. lA) [4]. Its aldehyde and acid forms are retinal (Fig. IB) and retinoic acid (Fig. 1C). The active form of vitamin A in vision is ll-cis retinal (Fig. ID), and a therapeutically useful form (accutane, isotretinoin) is l3-cis retinoic acid (Fig. IE). Retinyl palmitate (Fig. IF) is a major storage form, and retinoyl p-glucuronide is a biologically active, relatively non-toxic water-soluble metabolite (Fig. IG). A synthetic aromatic analog (etretin, acitretin), shows therapeutic usefulness (Fig. IH). Finally, p-carotene, a major provitamin A carotenoid, is shown in Figure II. 

Howard WB, Willhite CC (1986) Toxicity of retinoids in humans and animals. J Toxicol-Toxin Rev 5 55-94 Gunning DB, Barua AB, Olson JA (1993) Comparative teratogenicity and metabolism of al -trans retinoic acid, SiW-trans retinoyl p-glucose, and a -trans retinoyl p-glucuronide in pregnant Sprague-Dawley rats. 

Barua AB (1997) Retinoyl p-glucuronide a biologically active form of vitamin A. Nutr Rev 55 259-267 Formelli F, Barua AB, Olson JA (1996) Bioactivities of N-(4-hydroxyphenyl)retinamide and retinoyl p-glucuronide. FASEB y 10 1014-1024... 

Creech Kraft J, Slikker Jr, W, Bailey JR, Roberts LG, Fischer B, Wittfoht W, Nau H (1991) Plasma pharmacokinetics of 13-cw- and all-rraw5-retinoic acid in the Cynomolgus monkey and the identification of the conjugate metabolites 3-cis and all-rran -retinoyl-P-glucuronides a comparison to one human case study with Isotretinoin. Drug Metab Dispos 19 317-324... 

Creech Kraft J, Juchau MR (1992) Correlations between conceptal concentrations of 2 -trans retinoic acid and dysmorphogenesis after microinjections of all-fran5-retinoic acid, 13-c/5-retinoic acid, dX -trans-retinoyl-p-glucuronide or retinol in cultured whole rat embryos. Drug Metab Dispos 20 218-225... 

Creech Kraft J, Juchau MR (1992) Conceptal biotransformation of 4-oxo-all-rraw5-retinoic acid, 4-oxo-13-cw-retinoic acid and all-frans-retinoyl-p-glucuronide in rat whole embryo culture. Biochem Pharmacol 43 2289-2292... 

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