JBC, Vol. 250, Issue 24, 9342-9347, Dec, 1975
Patterns of fatty acid release from endogenous substrates by human platelet homogenates and membranes
A. Derksen and P. Cohen
We describe a method for measuring the release of fatty acids from
endogenous substrates of human platelet homogenates and membranes. The
method depends on the availability of lipids whose fatty acids are
odd-chained and therefore suitable as internal reference compounds that, at
the time of lipid extraction, can be added to an incubation to permit
subsequent quantification of the content of free fatty acids or fatty acids
esterified to specific lipids. We found four types of lipolytic activities
in human platelets. In homogenates at pH 4.0 a triglyceride lipase operated
as shown by the synchrony of triglyceride degradation and release of
glycerol and those fatty acids that are the predominant constituents of
triglycerides. However, enough arachidonic acid was released at this pH
level to suggest some phospholipid breakdown, since triglycerides hold
relatively small amounts of this acid. With membranous preparations, in the
alkaline pH range there were two peaks of fatty acid release with
accompanying degradation of phospholipids. At pH 8.5, where release of the
saturated acids, palmitic and stearic, predominated, their sum was 3.5
times that of arachidonic acid. At pH 9.5 the release of palmitic and
stearic acids was only slightly below their peak values; however, the
release of arachidonic acid nearly equaled the sum of the saturated acids.
Linoleic acid was not released in representative amounts by those reactions
that released arachidonic acid, despite the overwhelming propensity of both
to be esterified at the 2-position of phospholipids. Pertinently, the
choline phospholipids are linoleic-rich and the non-choline phospholipids
linoleic-poor, while both have a generous endowment of arachidonic acid.
With this in mind, we raise the possibility that the phospholipase A2 of
human platelets is an endoenzyme because of its tendency to act on those
phospholipids that are thought to comprise the inner layer of the cell
membrane.
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