Intestinal vitamin A metabolism: coordinate distribution of enzymes and CRBP(II)

Abstract

In the mucosal layer of the small intestine, we found nearly identical gradients of CRBP(II), retinal reductase, and LRAT levels down the duodenal-ileal axis, suggesting coordinate regulation of these three proteins. In all cases the level of binding protein or enzyme activity was greatest in the proximal intestine and then decreased sharply in the distal half. This pattern fits with the known capacity of the intestine to absorb vitamin A. In addition, the retinal reductase activity was found predominantly in the intestinal mucosa, while LRAT activity was 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 conjunction with LRAT activity and CRBP, we found endogenous retinyl ester stores in the intestinal muscle layer. The patterns of retinyl ester produced by LRAT in vitro and found in vivo were similar, with retinyl palmitate predominating and a high percentage comprised of retinyl stearate. We also observed a bile salt-independent retinyl ester hydrolase activity in intestinal muscle whose distribution paralleled the retinyl ester stores and LRAT levels. This hydrolase appears to be distinct 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 inhibited by diethyl-p-nitrophenyl-phosphate (IC50 100 microM) and diethylpyrocarbonate (IC50 10 microM), demonstrating a requirement for active serine and histidine residues. In addition, we describe an activity present in some intestinal microsomal preparations that can perturb determinations of reductase and LRAT activity and must be avoided.