Coordinated distribution patterns of three enzyme activities involved in the absorption and metabolism of β-carotene and vitamin a along the villus-crypt axis of chick duodenum

Abstract

The conversion of β-carotene to retinal and the succeeding metabolic process of the retinal leading to production of retinol and retinyl esters are the prerequisite for the utilization of β-carotene as a provitamin A. These processes are participated by β-carotene cleavage enzyme, retinal reductase and retinol esterifying enzyme(s) in the small intestine. To examine whether these enzymes exhibit the coordinated distribution in the villus, we have used the cryostat sectioning technique to quantify the activities of β-carotene cleavage enzyme, retinal reductase and retinol esterifying enzymes along the villus-crypt axis in 8-day-old chick duodenum. The β-carotene cleavage enzyme activity was very low in the crypt and gradually increased, reaching a maximum in the mid-villus. The villus-crypt gradient of the β-carotene cleavage enzyme activity corresponded with those of retinal reductase activity and lecithin: retinol acyltransferase (LRAT) activity, but distinct from that of acyl-CoA: retinol acyltransferase (ARAT) activity. Furthermore, the distribution of the content of retinyl esters was similar to that of LRAT activity. These results suggest that the β-carotene cleavage enzyme is coordinately distributed along the villus-crypt axis with retinal reductase and LRAT, the two enzymes which require cellular retinol-binding protein, typell (CRBPII) as the donor of the substrate.