scholarly journals Calcium Binding Sites on Human Blood Platelets

1977 ◽  
Author(s):  
S. Heptinstall
Keyword(s):  
Binding Sites ◽  
Calcium Binding ◽  
Blood Platelets ◽  
Human Platelets ◽  
Extracellular Calcium ◽  
Scatchard Analysis ◽  
Magnesium Ions ◽  
Binding Data ◽  
Calcium Binding Sites ◽  
Human Blood Platelets

Extracellular calcium ions are required for platelets to aggregate in response to various aggregating agents. Although magnesium ions can sometimes stimulate aggregation they only do so when a small amount of calcium is present. The calcium bound to washed human platelets suspended in buffered saline containing 0-200μM+5CaCl2 depends upon the extracellular calcium concentration. Scatchard analysis of the binding data suggests that a few (0,8 × 106) relatively high affinity (K = 85,000) calcium binding sites are present on each platelet. When 2.5mM MgCl2 is included in the saline suspensions the calcium bound to the platelets is only reduced at the higher calcium concentrations. Magnesium ions do not displace the tightly bound calcium. It is suggested that these specific calcium binding sites are involved in platelet aggregation.

The Binding Of 125I-Von Willebrand Factor To Human Platelets In The Presence Of Ristocetin

1981 ◽  
Author(s):  
P Silber ◽  
T H Finlay
Keyword(s):  
Von Willebrand Factor ◽  
Binding Sites ◽  
Binding Constant ◽  
Specific Binding ◽  
Human Platelets ◽  
Scatchard Analysis ◽  
Binding Data ◽  
Number Of Binding Sites ◽  
Von Willebrand ◽  
Willebrand Factor

The effect of ristocetin on the binding of 125I-porcine von Willebrand factor to human platelets was studied. Previously, we had shown that 125I-porcine von Willebrand factor binds to human platelets in the absence of ristocetin. The present work demonstrates that binding is stimulated by ristocetin and this stimulation is maximal at a ristocetin concentration of 2 mg/ml. At a ristocetin concentration of 0.5 mg/ml, Scatchard analysis indicates a binding constant of 5.18 × 10-9M and the presence of 105,000 binding sites. This compares with our previous finding, in the absence of ristocetin, of a binding constant of 2.92 × 10-7M and 4760 binding sites. These binding data assume the porcine von Willebrand factor to be a tetramer with a molecular weight of 9 × 105. This study indicates that ristocetin causes tighter binding and increases the number of binding sites on human platelets for porcine von Willebrand factor. Unlabelled porcine von Willebrand factor competitively inhibits the specific binding of the labelled protein and gives a binding constant of 0.17 × 10-9M. Similar results were obtained using human von Willebrand factor.

Interaction of β2-Glycoprotein-I with Human Blood Platelets: Influence Upon the ADP-Induced Aggregation

1985 ◽  
Vol 54 (02) ◽  
pp. 397-401 ◽  
Author(s):  
Johannes Nimpf ◽  
Helmut Wurm ◽  
Gerhard M Kostner
Keyword(s):  
Binding Sites ◽  
Specific Binding ◽  
Blood Platelets ◽  
Gel Filtration ◽  
Human Platelets ◽  
Glycoprotein I ◽  
Unspecific Binding ◽  
Induced Aggregation ◽  
Human Blood Platelets

SummaryThe interaction of β2-glycoprotein-I (β2-G-I), a plasma constituent of unknown function, with blood platelets was studied. The following results were obtained: 1) β2-G-I binds to washed human platelets isolated by centrifugation (WP) at one kind of specific, saturable binding sites. The dissociation constant was found to be approx. 1 × 10−6M.2) In the presence of physiological concentrations of Ca++ (2.5 mM), this specific binding is markedly reduced. Unspecific binding of β2-G-I to platelets, however, is not influenced by Ca++.3) Platelets prepared by gel filtration (GFP), differing in their in vitro aggregability from WP, exhibit no specific binding of β2-G-I. Binding to GFP is also not induced by activation with thrombin, collagen or ADP.4) β2-G-I causes significant alteration of the ADP-induced aggregation of GFP. Aggregation induced by thrombin, collagen, arachidonic acid or PAF-acether, however is not altered by β2G-I.It is suggested, that pelleting during centrifugation causes irreversible rearrangements in the membrane of platelets.

Identity of Saturable Calcium-Binding Sites on Blood Platelets and Their Involvement in Platelet Aggregation

1988 ◽  
Vol 59 (01) ◽  
pp. 054-061 ◽  
Author(s):  
Geoffrey I Johnston ◽  
Stanley Heptinstall
Keyword(s):  
Platelet Aggregation ◽  
Dissociation Constant ◽  
Binding Sites ◽  
Calcium Binding ◽  
Blood Platelets ◽  
Experimental Manipulation ◽  
Bleeding Disorder ◽  
Saturable Binding ◽  
Calcium Binding Sites ◽  
Full Occupancy

SummaryExtracellular Ca2+ ions are required for platelet aggregation and we show that they enter two platelet pools. One pool is rapidly filled and easily displaced by EGTA. The second is filled more slowly and is not displaced by EGTA. The EGTA-displaceable pool is believed to be surface-located and was found to contain at least one class of saturable binding sites as well as a class of non-saturable binding sites. The saturable sites were found to be highly selective for Ca2+ (dissociation constant, 3.5 × IO−7 M) even in the presence of 1 mM Mg2+ ions, and they took up between 261,000 and 307,000 Ca2+ ions/platelet. Full occupancy of the saturable binding sites appeared to be necessary for platelet aggregation to proceed. We also studied platelets that were unable to aggregate normally, either due to the congenital bleeding disorder Glanzmann’s thrombastenia or due to experimental manipulation. In both cases wc found decreased Ca2+uptake specifically by the saturable Ca2+ binding sites, and that this was associated with decreased number of GP IIb/IIIa molecules expressed on these platelets. We suggest that the Ca2+binding sites involved in platelet aggregation are located on the GP Ilb/IIIa complexes and may be involved in holding the glycoproteins in the complex together, and that the binding sites need to be fully occupied before aggregation can proceed.

scholarly journals The Number and Nature of ADP Receptors on Human Blood Platelets Determined with a Photoaffinity Label

1977 ◽  
Author(s):  
D.E. Macfarlane ◽  
D.C.B. Mills
Keyword(s):  
Binding Sites ◽  
Silicone Oil ◽  
Shape Change ◽  
Blood Platelets ◽  
Binding Data ◽  
Adp Receptors ◽  
Adp Receptor ◽  
Derivatives Of ◽  
Scatchard Plots ◽  
Human Blood Platelets

ADP interacts with platelets to cause shape change, aggregation and inhibition of adenylate cyclase. Previous attempts to measure the binding of ADP to its receptor on intact platelets have been frustrated by the low ratio of specifically bound ADP to the ADP trapped in centrifuged pellets. Several 2-substituted derivatives of ADP have higher apparent affinities for the ADP receptor, improving this ratio. We have prepared 2-azido 5'-ADP from 2-chloroadenosine in 20% yield. It was more active than ADP as an antagonist of PGE1-induced elevation of cyclic AMP (Ki = 96 nM, cf ADP Ki = 760 nM) and also as an inducer of shape change and aggregation. 2-Azido [β32P]-5'-ADP (2.5 Ci/mmole) was prepared and used to study binding. Platelets were separated from plasma by centrifugation through silicone oil (1.023 gm/ml) in a modified Eppendorf centrifuge. Scatchard plots of the binding data were resolved into two linear components, one of zero affinity, corresponding to trapped plasma, measured independently with [14C] sucrose, and a high affinity component with apparent KD = 110-160 nM. There were 400-700 of these binding sites per platelet. 2-Azido 5'-ADP is photolysable with light at <310 nm, forming a reactive nitrene potentially suitable for photoaffinity labelling. 2-Azido [β32P]-5'-ADP was photolysed in the presence of washed platelets which were subsequently rewashed, solubilized and electrophoresed on SDS-polyacrylamide gradient slabs. Several peaks of radioactivity were observed in the high molecular weight region of the gel (ca 100-250 kD) one of which was less labelled in the presence of excess ADP or ATP to block access of the label to the receptor.

Calcium and the Fc Receptor on Human Platelets

1987 ◽  
Vol 57 (03) ◽  
pp. 298-301
Author(s):  
William F Clark ◽  
Gerald J M Tevaarwerk ◽  
Bruce D Reid ◽  
Suzanne Hall ◽  
Anita Caveney ◽  
...  
Keyword(s):  
Specific Binding ◽  
Normal Subjects ◽  
Human Platelets ◽  
Fc Receptor ◽  
Normal Male ◽  
Scatchard Analysis ◽  
Normal Female ◽  
Apparent Difference ◽  
Binding Data ◽  
Direct Binding

SummaryWe have described the calcium dependence of the IgG Fc receptor (Fc-R) on human platelets by analyzing the direct binding of radiolabelled Fc fragments, monomers and dimers of IgG. Specific binding to platelets was undetectable at 37° C in a calcium-free preparation but readily detected when calcium was restored. Scatchard analysis of the binding data for the calcium-restored platelets permitted calculation of the available Fc-R and the Ka of binding for the different IgG ligands. The mean Ka of binding for 12 normal subjects varied from 107 to 108 L/M, with an equal receptor number measured by Fc fragments and dimers of IgG, but a lesser amount for monomeric IgG. There was no apparent difference in Fc-R number for platelets from 6 normal male versus 6 normal female subjects.At 4° C binding was detectable for dimers and polymers of IgG in a calcium-free preparation and this was markedly increased with recalcification. Thus, our data are consistent with an Fc receptor population on human platelets whose avidity for binding is significantly enhanced in a calcium-restored medium.

scholarly journals Mutation of the high affinity calcium binding sites in cardiac troponin C.

1992 ◽  
Vol 267 (2) ◽  
pp. 825-831 ◽  
Author(s):  
J C Negele ◽  
D G Dotson ◽  
W Liu ◽  
H L Sweeney ◽  
J A Putkey
Keyword(s):  
Binding Sites ◽  
Calcium Binding ◽  
Cardiac Troponin ◽  
Troponin C ◽  
High Affinity ◽  
Cardiac Troponin C ◽  
Calcium Binding Sites
Sign in / Sign up

Export Citation Format

Share Document