INTESTINAL VITAMIN-A METABOLISM - COORDINATE DISTRIBUTION OF ENZYMES AND CRBP(II)

In the mucosal layer of the small intestine, we found nearly identical gradients of CRBP(II), retinal reductase, and LRAT levels down the du odenal-ileal axis, suggesting coordinate regulation of these three pro teins. In all cases the level of binding protein or enzyme activity wa s greatest in the proximal intestine and then decreased sharply in the distal half. This pattern fits with the known capacity of the intesti ne to absorb vitamin A. In addition, the retinal reductase activity wa s found predominantly in the intestinal mucosa, while LRAT activity wa s found in both the intestinal mucosa and muscle. An even distribution of LRAT activity along the longitudinal axis of the intestinal muscle was consistent with an even distribution of CRBP in that tissue. In c onjunction with LRAT activity and CRBP, we found endogenous retinyl es ter stores in the intestinal muscle layer. The patterns of retinyl est er produced by LRAT in vitro and found in vivo were similar, with reti nyl palmitate predominating and a high percentage comprised of retinyl stearate. We also observed a bile salt-independent retinyl ester hydr olase activity in intestinal muscle whose distribution paralleled the retinyl ester stores and LRAT levels. This hydrolase appears to be dis tinct from retinyl ester hydrolases described from other organs as its activity was insensitive to retinyl ester chain length, the presence of bile salts, or the addition of apo-CRBP. This activity was inhibite d by diethyl-p-nitrophenylphosphate (IC50 100 mum) and diethylpyrocarb onate (IC50 10 muM), demonstrating a requirement for active serine and histidine residues. In addition, we describe an activity present in s ome intestinal microsomal preparations that can perturb determinations of reductase and LRAT activity and must be avoided.