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Lipases and Carboxylesterases: Possible Roles in the Hepatic Utilization of Vitamin A^{1 ,2}

Human Nutrition Research Center, U.S. Department of Agriculture, Agricultural Research Service, Beltsville, MD 20705

The formation and hydrolysis of retinyl esters are key processes in the metabolism of the fat-soluble micronutrient vitamin A. Long-chain acyl esters of retinol are the major chemical form of vitamin A (retinoid) stored in the body. Although retinyl esters are found in a variety of tissues and cell types, most of the total body retinoid is accounted for by the retinyl esters stored in the liver. Thus, these esters represent the major endogenous source of retinoid that can be delivered to peripheral tissues for conversion to biologically active forms. This paper summarizes the current state of our knowledge about the identity, function and regulation of the hepatic enzymes that are potentially involved in catalyzing the hydrolysis of retinyl esters. These enzymes include several known and characterized lipases and carboxylesterases.

KEY WORDS: • retinoids • retinol • retinol esters • hydrolases • liver