Intra- and extracellular co-compartmentalization of newly
synthesized rat intestinal alkaline phosphatase and a surfactant-like
particle protein.
Alpers, D. H., Zhang, Y, and Ahnen, Dj.
Division of Gastroenterology, Washington University School of
Medicine, St. Louis, MO, and University of Colorado Health Sciences
Center, Department of Veteran Affairs, Denver, CO
APStracts 2:0034E, 1995.
Two rat intestinal alkaline phosphatases are directed to the
microvillus membrane and also appear to be secreted bidirectionally
from the enterocyte attached to a phospholipid-rich membrane
(surfactant-like particle). To determine the intracellular pathways
for both newly synthesized alkaline phosphatases and for the
extracellular enzyme-particle complex in the intestinal mucosa,
pulse-chase experiments with radioactive amino acid precursors were
performed. Synthesis of both isomers of alkaline phosphatase in
fasted rats peaked in the Golgi fraction at 15-30 min., and in the
microvillus membrane at 60 min., without an apparent intermediate
localization in the basolateral membranes. An early second peak of
incorporation in IAP was found at 15-30 min. in the luminal scrapings
obtained from the apical surface of the enterocytes. These results
are consistent with a dominant direct Golgi to microvillus membrane
transport for newly synthesized intracellular alkaline phosphatase,
and with at least one additional precursor pool that is responsible
for the early appearance of enzyme in the luminal washings.
Incorporation over 30 min. of labelled precursor into various mucosal
fractions also was determined at intervals after fat feeding. Newly
synthesized alkaline phosphatase isomers and the 97 kDa protein of
surfactant-like particles showed similar patterns of appearance in
enterocytes, luminal washings, and lamina propria after
triacylglycerol feeding, but continued to be preferentially secreted
into the lumen and lamina propria at times (5-7 h) when enterocyte
content of these newly-synthesized proteins had declined toward basal
rates. These data are consistent with parallel secretion of both
enzyme isomers and the particle into the tissue compartments.
Moreover, enhanced secretion of the newly synthesized proteins for
hours after fat feeding could explain the prolonged rise in serum and
lumenal washings of both the enzyme and the particle.
Received 29 August 1994; accepted in final form 6 January 1995.
APS Manuscript Number E348-4.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 1 March 1995.