Ontogeny of two vitamin A-metabolizing enzymes and two retinol-binding proteins present in the small intestine of the rat.

The patterns of expression of cellular retinol-binding protein (CRBP), cellular retinol-binding protein, type two [CRBP(II)], lecithin: retinol acyltransferase (LRAT), and microsomal retinal reductase were examined for rat small intestine during the perinatal period. CRBP was present (15 pmole per mg soluble protein) at the earliest time examined, the 16th day of gestation, declining by 70% by birth, maintained to adulthood. In contrast, CRBP(II) appeared 2-3 days before birth, rising to its highest level (500 pmole per mg soluble protein) by day 3 after birth, then declining by 50% during the late suckling period to the adult level. Immunohistochemistry revealed that CRBP(II) initially appeared in the epithelial cell layer in a patchy manner, resolving by birth into an even staining of all villus-associated enterocytes. In contrast, CRBP was evenly expressed in the epithelial cell layer at day 17/18 but was absent by birth. Intestinal LRAT activity increased rapidly in the 2 days prior to birth, then declined at weaning to the adult level. Microsomal retinal reductase was measurable in the intestine at birth, but not detected during the early suckling period, reappearing at day 21. Considerable increase was then observed coincident with weaning, when carotenes, from which retinal is derived, became an important source of vitamin A. The pattern of appearance of these elements appears to prepare the intestine for the necessary processing of vitamin A required after birth.