Retinol esterification in the hepatopancreas of the crayfish (Procambarus clarkii)

Abstract

Vitamin A (retinol) is a necessary nutrient for all higher animals, including many invertebrates which use it as their visual chromophore. All of these animals must get vitamin A in their diet and must be able not only to transport this hydrophobic vitamin to target cells but must also convert it into active forms of the vitamin, such as retinal and retinoic acid. In mammals, retinol is converted into retinyl esters for export from the small intestine, for storage in the liver, and as part of the visual cycle. Little is known about these processes in invertebrates. Mammals have two enzymes that can catalyze the conversion of retinol to retinyl‐esters: lecithin:retinol acyltransferase (LRAT) and acyl‐coA:retinol acyltransferase (ARAT). LRAT is the physiologically important one in mammals. We have identified an ARAT activity in microsomes from the hepatopancrease of the freshwater crayfish, Procambarus clarkii. We were unable to detect any LRAT activity, suggesting crayfish only have one enzyme that can catalyze this reaction. This activity shows a linear dependence on time and microsome concentration. It shows temperature dependence and an optimal pH of 6.5. We have characterized its acyl‐CoA preference and determined Km and Vmax's for both retinol and the acyl‐CoA.