Retinoid glucuronides

The mechanism of action for retinoid glucuronides is not established [192]. A novel action of the conjugates is suggested, because the glucuronides do not bind to cellular retinoic acidbinding proteins or to nuclear receptors for retinoic acid, although they are active prior to their... 

The higher concentration reported by Napoli et al. (22) may be due to hydrolysis of the glucuronide metabolite since retinoyl glucuronide is hydrolyzed by acid or base to retinoic acid (33.341. The extraction procedure of Napoli et al. (22) utilized 2N HC1 which could potentially convert most of the retinoyl glucuronide to retinoic acid. Therefore, the concentration of all trans retinoic acid as reported by this assay could represent the sum of these two retinoids, and not that of the free acid (22). Our assay uses less harsh acidic conditions for extraction which should prevent hydrolysis of the glucuronide, resulting in a lower plasma concentration of the all trans isomer as reported by Napoli et al. 22). 

Topical retinoids—tretinoin, adapalene, tazarotene, and in some countries topical isotretinoin, motretinide, retinaldehyde, and retinoyl-/S-glucuronide —should be used as first-line therapy for mild to moderate inflammatory acne and comedonal acne. They are also preferred agents for maintenance therapy to minimize antibiotic use in acne therapy. To optimize efficacy in moderate inflammatory acne, topical retinoids should be combined with topical antibiotics or BP0.2T33... 

M. J. Robarge, J. J. Repa, K. K. Hanson, S. Seth, M. Clagett-Dame, H. Abou-Issa, and R. W. Curley Jr., N-linked analogs of retinoid 0-glucuronides Potential cancer chemopreventive/ chemotherapeutic agents, Bioorg. Med. Chem. Lett., 4 (1994) 2117-2122. 

A series of 14 retinoids, carotenoids, and tocopherols from very polar (e.g., all-rranj-4-oxo-retinoyl /J-glucuronide, all-rranj-4 oxo-retinoic acid, all-rrans-5,6-epox-yretinoic acid) to slightly polar (e.g., all-tranj-retinol, all-trans-retinoic acid) to nonpolar (e.g., all-fra 5-lycopene, all-rrans-jS-carotene) were resolved on a Micro-sorb-MV column (A = 330nm) using a 100/0->0/100 (at 15 min hold 19 min) (3/1 methanol/water with 10mM ammonium acetate)/(4/l methanol/chloroform) gradient [350]. 

Figure 1 Some naturally occuring retinoids (a) all-fra/ii -retinol (b) all-frans-retinal (c) all-fran5 -retinoic acid (d) 13-cw-retinoic acid (e) all-frani-3,4-didehydroretinol (vitamin Aj alcohol) (f) 11-cw-retinal (g) all-franj-5,6-epoxyretinol (h) all-irani-anhydroret-inol (i) all-rran5 -4-oxoretinol (j) all-irans-retinoyl 5-glucuronide (k) all-frani-retinyl i-glucuronide (1) all-franj-retinyl palmitate.

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. 

The predominant retinoid in the fasting circulation is retinol, all of which is bound to its specific plasma transport protein, retinol-binding protein (RBP) [1,2]. Although retinol accounts for approximately 95 to 99% of all retinoid in the circulation, other retinoids also are present. Fasting human and rodent blood contains very low levels of both sAhtrans- and 13-cw-retinoic acid (approximately 0.2 to 0.7% of those of retinol) [3], as well as low levels of retinyl esters in lipoprotein fractions, particularly very low-density lipoproteins (VLDL) and low density lipoproteins (LDL) [4]. Soluble glucuronides of both retinol and retinoic acid are also detectable in the circulation of humans and rodents [5], as are provitamin A carotenoids like P-carotene... 

Following consumption of a retinoid-rich meal, the circulation can also contain large amounts of retinyl esters in chylomicrons and/or their remnants [6,7]. Postprandial retinyl ester concentrations can easily exceed fasting retinol levels by several fold [7, 8]. Moreover, consumption of a retinoid-rich meal brings about an increase in circulating concentrations of all-trans- and 13-cw retinoic acid and of glucuronides of both retinol and retinoic acid [7, 9]. 

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. 

It should be emphasized that investigations of embryonic enzymatic catalysis of the four retinoid biotransforming reactions (2ill-trans/9-cis isomerization, retinol and retinal dehydrogenation, glucuronidation and monooxygenation) have been numerous [6,7,10-13,15,18-22, 77-85]. 

Abou-Issa H, Curley RW Jr, Panigot MJ, Tanagho SN, Sidhu BS, Alshafie GA (1997) Chemotherapeutic evaluation of N-(4-hydroxyphenyl) retinoid 0-glucuronide in the rat mammary tumor model. Anticancer Res 17 3335-3340... 

Balakrishnan V, Gilbert NE, Brueggemeier, Curley RW Jr (1997) N-linked glycoside/glucuronide conjugates of retinoids Acitretin. Bioorg Med Chem Lett 7 3033-3038... 

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