Photoaffinity labeling of bile acid receptors in the mucosa have ident
ified a 93-kD apical membranous protein and a 14-kD cytosolic protein,
whereas using an expression cloning strategy, a complementary DNA clo
ne encoding a predicted 38-kD protein has been identified. A lipoprote
in with surfactant-like properties has been shown to be associated wit
h lipid export from the intestine. Lipid transport can be blocked by t
he in vivo administration of Brefeldin A. A neutral and basic amino ac
id transporter was identified in the brush border of intestinal cells
that is important in taurine absorption. Dipeptidyl peptidase IV was s
hown to be physiologically important. Tyrosine kinase was suggested to
be important in Na+-H+ exchange at the apical surface and NaCl absorp
tion basolaterally. There is an excess of lactase messenger RNA compar
ed with activity during the maturation process. Fasting/refeeding expe
riments activate the early response genes c-fos and c-jun.