Platelet multielemental composition, lability, and subcellular localization

Abstract

Diagnostic X-ray spectrometry (DXS), based on X-ray fluorescence, was used to quantitate directly the multiple elemental composition of washed, intact human platelets (n = 16), with the following results: K = 3.08 +/- 1.00 mg/g, Ca = 1.18 +/- 0.29 mg/g, Zn = 35 +/- 9 micrograms/g. These values show that washed platelets contain significant pools of K, Ca, and Zn, the latter some 30-60-fold higher than plasma levels. Dialysis of whole platelets against cation exchange resin (Chelex-100) did not extract Ca(II) and Zn(II) sequestered within whole cells. To identify the subcellular locale of the elements, platelet lysate was subjected to 30-70% sucrose gradient ultracentrifugation and subcellular enriched fractions were obtained. Fractions were analyzed by DXS (for elements), electron microscopy (for dense granules), and subcellular markers fibrinogen and von Willebrand factor. In contrast to Ca and K, which accumulate in the dense granules and the cytoplasm, respectively, Zn appears to be distributed in the alpha-granules (40%) and the cytoplasm (60%). The subcellular distribution of Zn(II) is discussed within the context of the sensitivity of platelet response to the availability of Zn(II) and the platelet release reactions following stimulation.

Plasminogen Is Tethered with High Affinity to the Cell Surface by the Plasma Protein, Histidine-rich Glycoprotein

“…1). In this regard, Zn 2ϩ stored in platelet granules is released rapidly into the plasma after thrombin-stimulated platelet degranulation (47,48), while pH can drop as much as one pH unit during hypoxia or ischemia, or 0.5 of a pH unit during an inflammatory response because of lactic acidosis. Thus, at sites of inflammation, angiogenesis, and wound healing, acidic pH and elevated free Zn 2ϩ provide a mechanism for plasmin activity to be expressed selectively on cell surfaces.…”

Differential Binding of Histidine-rich Glycoprotein (HRG) to Human IgG Subclasses and IgG Molecules Containing κ and λ Light Chains

“…In fact, Zn 2ϩ further accentuated the influence of the L-chain isotype on HRG-IgG1 binding, with the K d of 0.6 nM and 266 nM (a 443-fold difference) for the binding of HRG to IgG1 and IgG1, respectively. Zn 2ϩ is an interesting regulator of the IgG1-HRG interaction, and the level of Zn 2ϩ is reported to vary in tissues (28) with relatively large amounts of Zn 2ϩ being released locally by degranulating platelets (28,29). The question arises, therefore, as to whether the findings on the effects of Zn 2ϩ are physiologically relevant.…”

“…Another important implication of the present work follows from the fact that both soluble and insoluble ICs can be generated during an immune response to an antigen and that IICs are likely to be important initiators of localized inflammatory reactions to pathogens (17,19 (19,29) may serve as long-lasting depots of antigen for the maintenance of immunological memory. The ability of IgG molecules to interact with HRG and consequently form soluble versus insoluble ICs depending on the L-chain isotype they express therefore also suggests functional differences in the role of the and L-chains in regulating this process.…”

Zinc Modulates the Interaction of Protein C and Activated Protein C with Endothelial Cell Protein C Receptor

“…In addition, platelets store, mainly in their cytoplasm and ␣-granules, up to 30 to 60 times (14) more Zn 2ϩ than is present in plasma. These reserves could be released upon platelet activation (14).…”

“…Zinc is an essential trace element, and the concentration of Zn 2ϩ in plasma ranges between 10 and 25 M in healthy individuals (13). In addition, platelets store, mainly in their cytoplasm and ␣-granules, up to 30 to 60 times (14) more Zn 2ϩ than is present in plasma. These reserves could be released upon platelet activation (14).…”