Hydrolysis of membrane-bound liver alk aline phosphatase by gpi-pld
requires bile salts.
Deng, Jing T., Marc F. Hoylaerts, Marc E. De Broe, Viviane O. Van
Hoof.
Department of Nephrology-Hypertension, University of Antwerp,
Belgium, Department of Clinical Chemistry, University Hospital,
Antwerp, Belgium
APStracts 2:0261G, 1995.
Circulating liver plasma membrane fragments (LiPMemF) were purified
from human serum serum by means of a monoclonal anti-leucine
aminopeptidase antibody AD-1. This was done by immunoaffinity
chromatography or by incubating the sera with AD-1 coated
nitrocellulose disks. Alkaline phosphatase (ALP, E.C.3.1.3.1.) is
bound to these LiPMemF through a glycosyl-phosphatidylinositol (GPI)
-anchor and is referred to as Mem-LiALP. Low concentrations of Triton
X-100, or high bile salt concentrations, released GPI-bearing liver
ALP (Anch-LiALP) from purified LiPMemF ; once released, Anch-LiALP
was slowly and progressively converted to hydrophilic dimeric liver
ALP (Sol-LiALP), free from its GPI-anchor. Low levels of GPI specific
phospholipase-D (GPI-PLD) activity were measured in the pure LiPMemF.
Apparently, this membrane associated GPI-PLD was released by the
action of detergents and contributed to the 'spontaneous' conversion
of Anch-LiALP to Sol-LiALP. In the absence of detergents, GPI-PLD had
little effect on Mem-LiALP, both in purified form as well as in
serum. In vitro, isolated Anch-LiALP was converted to Sol-LiALP by
both GPI-specific phospholipase C (GPI-PLC) and GPI-PLD. Sol-LiALP in
serum however appeared to be the product of GPI-PLD activity only.
Five to ten-fold higher con centrations of Triton X-100 were needed
to release Anch-LiALP from LiPMemF in serum, as compared to those
required in a solution of purified LiPMemF. In serum as well as in
purified conditions, only a small window of detergent or bile salt
concentrations permitted the conversion of Mem-LiALP to Sol-LiALP. A
model for the release in the circulation of Mem-LiALP, Anch-LiALP and
Sol-LiALP, involving both LiPMemF associated GPI-PLD and liver
sinusoid bile salts is proposed.
Received 10 April 1995; accepted in final form 6 December 1995.
APS Manuscript Number G150-5.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 December 95