Intestinal alkaline phosphatase is secreted bidirectionally from villous enterocytes.
A fraction of intestinal alkaline phosphatase (IAP) is secreted into blood. To study this process, enzyme secretion was examined in a fetal (IRD-98) and a differentiated (Caco-2) intestinal cell line. Tissue-unspecific alkaline phosphatase (AP) activity in the IRD-98 cells increased 20-fold after addition of 1.5 mM sodium butyrate and 40 mM NaCl, but no AP activity was secreted into the medium. In contrast, newly synthesized IAP in Caco-2 cells was secreted into the medium. AP secretion increased with time and was inhibited by monensin. Medium AP was still partially bound to membranes as assessed by Triton X-114 phase separation and could be released by the addition of serum. Analysis by sodium dodecyl sulfate polyacrylamide gels and by isoelectric focussing showed that secreted AP gave a pattern similar to that of the AP released from membranes by phospholipase D treatment. When Caco-2 cells were grown on filters, AP activity was found in both basolateral (75%) and luminal (25%) media. These data demonstrate that the secretion of a particulate AP with extracellular release from the membrane can account for the appearance of the intestinal isozyme in both the serum and the lumen.
Sussman
N L
NL
Gastroenterology Division, Baylor College of Medicine, Houston, Texas 77030.
Eliakim
R
R
Rubin
D
D
Perlmutter
D H
DH
DeSchryver-Kecskemeti
K
K
Alpers
D H
DH
D000230:Adenocarcinoma; D000469:Alkaline Phosphatase / Q000378:metabolism*; D000818:Animals; D002087:Butyrates / Q000494:pharmacology; D020148:Butyric Acid; D002454:Cell Differentiation; D002460:Cell Line; D003110:Colonic Neoplasms; D004386:Duodenum / Q000187:drug effects / Q000196:embryology / Q000201:enzymology; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007082:Ileum / Q000187:drug effects / Q000201:enzymology; D007422:Intestines / Q000187:drug effects / Q000201:enzymology*; D007525:Isoelectric Focusing; D008871:Microvilli / Q000201:enzymology; D008985:Monensin / Q000494:pharmacology; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D010739:Phospholipase D / Q000378:metabolism / Q000494:pharmacology; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D051381:Rats; D012965:Sodium Chloride / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D002087:Butyrates; D020148:Butyric Acid; D012965:Sodium Chloride; D008985:Monensin; D000469:Alkaline Phosphatase; D010727:Phosphoric Diester Hydrolases; D010739:Phospholipase D; D043265:Phosphatidylinositol Diacylglycerol-Lyase