Glucuronic acid, conjugation with retinoic

Numerous metabolites of retinol or retinoic acid are more polar than retinol or retinoic acid due to additional oxidation of the cyclohexenyl ring, often on carbon 4. Some retinoids, particularly retinoic acid and 4-keto-retinoic acid, may be conjugated with glucuronic acid, forming retinyl- or retinoyl-/3-glucuronide (R5) these metabolites are water-soluble and therefore readily excreted. While some polar and water-soluble retinoids possess bioactivity, most show reduced, or no, activity compared to their precursors. 

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