5.6- Epoxyretinoic acid

Panzella, L, Manini, P, Napolitano, A, and d Ischia, M, 2004. Free radical oxidation of (E)-retinoic acid by the Fenton reagent Competing epoxidation and oxidative breakdown pathways and novel products of 5,6-epoxyretinoic acid transformation. ChemRes Toxicol 17, 1716-1724. 

Both all-frans and 13-cz s-retinoic acids as well as 5,6-epoxyretinoic acid are found in tissue.f°,p,pq... 

McCormick, A.M., Napoli, J.L., Schnoes, H.K., DeLuca, H.F. 1978. Isolation and identification of 5,6-epoxyretinoic acid a biologically active metabolite of retinoic acid. Biochemistry 17, 4085 1090. 

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. 

In the normal-phase HPLC, compounds to be separated are adsorbed to microparticulate silica gel and eluted in the order of least polar to most polar. Acceptable separation and quantitative yields of neutral and charged retinoids are obtained. Reverse-phase HPLC is preferable for acid-sensitive compounds such as 5,6-epoxyretinoic acid. Photometric, electrochemical, and mass spectrophotometric detectors have been used. 

Oxidation of retinol produces retinoic acid tretinoin). The reaction is irreversible. Retinoic acid enters the portal blood, is transported bound to albumin, and is not stored to any great extent. The concentration of retinoic acid in plasma is normally 3-4 ng/mL. A biologically active metabolite, 5,6-epoxyretinoic acid, has been isolated from the intestinal mucosa of vitamin A-deficient rats following administration of H-retinoic acid. Several tissues have specific cellular retinoic acid-binding proteins (CRABPs). 

Retinoic acid 5,6-epoxidation was catalysed by human blood cells but not by human plasma, and purified human haemoglobin also catalysed the ep-oxidation of retinoic acid to 5,6-epoxyretinoic acid (Iwahashi et al. 1985). 

Saponification of Methyi Esters to 4-Oxoretinoic Acid, 4-Hydroxyretinoic Acid, and 5,6-Epoxyretinoic Acid... 

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