Abstract
INTRODUCTION. Cell-derived microparticles (MP) such as from platelets (PMP), endothelia (EMP) and leukocytes (LMP) are increasingly recognized as useful biomarker and important mediators of thrombosis and inflammation. However, little attention has been paid to the possible role of MP from RBC (RMP) in vascular disorders. RMP were identified by glycophorin (GPH) in flow cytometry in most studies. We reported heterogeneity of RMP in size and phenotypes and that GPH is expressed predominately in larger RMP, not in smaller RMP and that GPH+ RMP are more active than GPH- RMP in thrombin generation. Since acetylcholinesterase (AChE) activity has been measured on RMP, and was recently proposed as a marker of some inflammatory states, we investigated AChE activity of RMP compared to platelet-derived MP (PMP). AChE of PMP has not previously been reported.
METHODS. RMP were prepared from intact washed RBC at 18% Ht exposed to calcium ionophore (4uM) in presence of calcium (2mM) for 30 min. PMP were prepared from 20 mL citrated blood, and exposing the platelet-rich plasma to 1 uM calcium ionophore (without added Ca2+) and collagen, 4ug/mL, for 20 min. AChE assay was based on Ellman’s method and reagent (DTNB), run in 96-well plates, 300uL. Substrate was acetylthiocholine iodide (1 mM f.c.). DTNB was used at 0.67 mM f.c. Tests were run +/− quinidine (Q) (1.2 uM) and some tests were in presence of saponin 0.01%. Q is known to inhibit AChE of plasma but RBC activity is insensitive. Activity is expressed in umols substrate cleaved /min per 108 MP, with provisos below. Flow cytometry using FITC labeled lectin, Ulex europaeus (Ulex) was used to quantitate RMP and PMP.
RESULTS.
As expected, Q inhibited AChE in plasma by >90% but not AChE of RMP. On contrary, RMP were consistently stimulated by Q, up to 150% activity +Q; some preparations of PMP were also stimulated. Saponin, which has been used in assay of RBC AChE, had little effect on PMP or RMP activity. In 12 experiments, AChE of PMP exhibited marked concentration-dependence. The apparent activity per mL of suspension was greater with lesser volumes, by as much as 3-fold between 2.5uL and 20uL added. This could not be explained by substrate inhibition since the effect varied in different preparations, was absent in particle-free plasma, and did not diminish in low substrate. This suggests the presence of a natural inhibitor. Calculation of specific activity of the MP was complicated by the dependence of apparent activity on volume assayed. However, when equal dilutions were compared, a representative experiment showed RMP had about 6-fold greater activity than PMP per 108 MP: 36.0 vs. 5.88 for 2.5uL suspension; and 29.0 vs. 3.9 for 20 uL assayed, in units above.
CONCLUSIONS / DISCUSSION. The AChE activity of RMP is about 6-fold greater than PMP. Weaker activity on PMP is possibly attributed to a previously unreported natural inhibitor. Blood AChE activity has been shown to reflect inflammatory states. Since AChE is a GPI-anchored protein, it is preferentially depleted from cells on the MP shed off. Assay of this activity in patient cell-free plasma, +/− Q, may be a useful biomarker. It is well known that hemolytic anemia, where RMP are elevated, is often associated with thrombotic complications, whereas ITP, where PMP are frequently elevated, rarely is. Further study to characterize AChE in RMP and other MP, and to clarify the physiological role of MP- and cell-associated AChE in thrombosis, inflammation, and cardiovascular disease is in progress.
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