J. Biol. Chem., Vol. 262, Issue 19, 9293-9297, 07, 1987
Two different Ca2+ transport systems are associated with plasma and intracellular human platelet membranes
J Enouf, R Bredoux, N Bourdeau and S Levy-Toledano
Platelet plasma and intracellular membrane fractions were isolated from a
mixed membrane fraction after sucrose cushion centrifugation. Their
previous identification through biochemical and immunological
characterization is now confirmed by sodium dodecyl sulfate electrophoresis
of the membrane proteins which reveals a different protein profile. The two
associated calcium transport systems showed a different time course and
exhibited different oxalate sensitivity. The plasma membranes are not
permeable to potassium oxalate but the Ca2+ uptake was stimulated by
potassium oxalate in intracellular membranes. We then focused on the study
of the plasma membrane-associated Ca2+- activated ATPase which shows the
following characteristics: a linearity in the time course until 30 min, an
apparent affinity toward calcium of about 10(-7) M without detectable
inhibition at higher concentrations, a maximal activity at pH 8, a high ATP
requirement because the maximal response was obtained with 200 microM, and
a high specificity toward ATP as energy donor. Taken together, these
studies indicate the possible involvement of both a plasma membrane and a
dense tubular system Ca2+-ATPase in the regulation of Ca2+ concentration in
human platelets.
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