Platelet lysosomal acid phosphatase enzyme activity as a marker of platelet procoagulant activity.

Abstract

Dear Sir, platelet lysosomal acid phosphatase has a unique property: it is always firmly bound to its phospholipoprotein carrier. It cannot be solubilised even in the presence of non-ionic detergent Triton X-1001. This phospholipoprotein appears to be associated with platelet procoagulant activity (PF3)2. A review of biochemical studies revealed that acid phosphatase activity could be found in platelet lysosomes as well as in platelet a granules3. During platelet activation, an increase in detectable platelet acid phosphatase activity thus also indicates the appearance of this lysosomal phospholipoprotein and its function. In order to display their hidden lysosomal acid phosphatase activities as well as procoagulant activities, platelet secretory lysosomes must obviously undergo exocytosis and calcium-dependent disintegration-lipolysis. This platelet exocytosis is a matter of some controversy. Basically three modes of exocytosis had been proposed based on transmission electron microscopy studies4,5. In our own published and unpublished experiments it was established that whenever there was detectable increase in platelet procoagulant activity, there was an increase in detectable platelet acid phosphatase, when using kaolin, collagen, thrombin or immune complexes as agonists3. Dissociation of these two activities in activated platelets was never observed. Fig. 1 shows an example of detectable acid phosphatase activity in relation to Stypven time in a suspension of kaolin-activated platelets at different times of activation between 0’ and 30’. After 30’ of activation approximately 10%–15% of the total platelet acid phosphatase lysosomal activity could be detected under conditions of citrated platelet-rich plasma. The major part of this increased acid phosphatase activity was sedimentable with platelet aggregates, suggesting it may also be involved in the formation of coated platelets. Figure 1 Acid phosphatase activity plotted against precoagulant activity as measured by Stypven time. As described elsewhere5, blood platelets from patients with Scott syndrome do not show increased procoagulant activity and microparticle formation upon activation; it would be interesting to know whether, correspondingly, they would also fail to display increased platelet lysosomal acid phosphatase activity upon activation. It seems that the relationship between platelet procoagulant activity and platelet lysosomal acid phosphatase activity offers an alternative view of some basic concepts in platelet pathophysiology.