Retinoic acid enterohepatic circulation

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. 

Zackman, R. D., Dunagin, P. E., and Olson, J. A. (1966). Formation and enterohepatic circulation of metabolites of retinol and retinoic acid in bile duct-canulated rats. J. Lipid Res. 7, 3-9. 

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