Subcellular distribution and characterization of acetylcholinesterase
activities from sheep platelets: relationships between temperature-
dependence and environment
J Sanchez-Yague, JA Cabezas and M Llanillo
Department of Biochemistry and Molecular Biology, Faculty of Biology,
University of Salamanca, Spain.
Acetylcholinesterase is a key enzyme in cholinergic neurotransmission for
hydrolyzing acetylcholine and has been shown to possess arylacylamidase
activity in addition to esterase activity. The enzyme is found at various
loci, where its functional significance remains to be clarified, and it
exists in multiple molecular forms. Sheep platelets have been shown to
exhibit acetylcholinesterase activity associated with plasma membrane (Bp),
endoplasmic reticulum (Cp), mitochondria granules (Dp), and soluble (As)
fractions. These activities show differences in some physicochemical and
kinetic properties. The soluble acetylcholinesterase is the most
thermostable, and the enzyme from the Cp fractions shows the lowest
affinity for the acetylthiocholine substrate and the strongest inhibition
by fluoride. In all cases a noncompetitive inhibition of the enzyme by this
ion is found. When membrane-bound acetylcholinesterases were assayed at
temperatures between 12 degrees C and 33 degrees C, the Arrhenius plots of
all activities exhibited a break point at about 17 degrees C. This
discontinuity was abolished by addition of detergent to the assay medium
(0.02% Triton X-100, final concentration). Their Hill coefficients were
calculated in the presence of fluoride, showing unitary values in all
cases, which points to a noncooperative effect and nonallosteric behavior
in the particulate enzyme. These results suggest that the sheep platelet
acetylcholinesterase associated with membrane-bound systems is modulated by
the physical state of its environment, despite the fact that the enzyme
might be lipid- or nonlipid-dependent.
Volume 76,
Issue 4,
pp. 737-744,
08/15/1990
Copyright © 1990 by The American Society of Hematology
