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Biomedical subjects

U Till

Publications and source records attributed to U Till.

At least 73 records · Page 4Linked to original sources
PubMed1987

Changes in the distribution and organization of platelet actin induced by diamide and its functional consequences.

Exposure of blood platelets to diamide (azodicarboxylic acid-bis-dimethylamide) results in oxidation of sulphydryl groups present in the cytoskeleton and other proteins. This results in dramatic changes in functional behaviour of the cells. The distribution and level of organization of the major cytoskeletal protein actin has been studied analytically by the DNase-I inhibition assay and morphologically by electron microscopy (EM) of Triton X-100 treated platelets adherent to EM grids. Exposure to diamide results in a redistribution of actin within the cell reflected in an increase in cytoskeletal F-actin and a concomitant decrease in cytosolic actin. The magnitude of these changes depends upon the concentration of diamide and the time of exposure. Diamide also alters platelet aggregatory functions in response to certain stimuli. Treatment of normal human platelets with 0.1 mM diamide proceeds via disaggregation (5 min exposure to diamide), inhibition of aggregation (30 min exposure), to finally a normalization of the aggregation response after 60-120 min incubation with diamide. In parallel with the return to full functional response the distribution of F-actin between the cytoskeleton and cytoplasmic compartments returns to the control pattern. Incubation of the platelets with 0.5 mM diamide for 60 or more minutes leads to total inhibition of the aggregatory ability. In these cells the cytoskeleton associated F-actin remains significantly elevated and the structural organization of the cytoskeleton is markedly altered. In contrast to the network of filaments subadjacent to the surface membrane seen in unstimulated platelets, the cytoskeleton now shows electron dense zones in the more central parts of the cytoplasm. This diamide-induced structural reorganization of platelet cytoskeletal elements, associated with the inhibition of functional responses, emphasizes the dynamic nature of the membrane-cytoskeletal axis and its importance in the expression of shape changes and aggregatory phenomena in response to surface stimuli.

Actins↗
PubMed1986

The role of lipoproteins in the degradation of platelet-activating factor.

Measuring both platelet-stimulating activity and liberation of acetate, the capacity of serum and individual lipoproteins to degrade the platelet-activating factor (PAF) was studied. The highest degrading effect relative to the protein content was found in very low density lipoproteins (VLDL) and in low density lipoproteins (LDL). The effect is about 10- and 100-fold higher than that of high density lipoproteins (HDL) and serum, respectively. In lipoprotein deficient serum (LPDS) less than 5% of serum activity is detectable. Considerable individual variations are observed measuring the degradation of PAF under standard conditions in plasma from 37 healthy volunteers. Moreover, this activity is shown to correlate strongly with the plasma concentration of LDL. On the other hand, a significant negative correlation was found between the PAF-degrading capacity and the plasma concentration of HDL. In contrast, the PAF-degradation is unrelated to the concentration of plasma triglycerides. The results point to a possible role of plasma lipoproteins in regulating the degradation of PAF released into the circulation.

Humans↗
PubMed1986

Prothrombinase activity of human platelets is inhibited by beta 2-glycoprotein-I.

In the present paper the influence of beta 2-glycoprotein-I, also known as apolipoprotein H, upon the prothrombinase activity of platelets and phospholipid vesicles was investigated. The results can be summarized as follows. 1. The prothrombinase activity of resting, non-activated platelets, lysed platelets and vesicles composed of phosphatidylserine and phosphatidylcholine at different molar ratios is inhibited by beta 2-glycoprotein-I in a dose-dependent manner. The concentration of glycoprotein which produces marked inhibition is within the physiological plasma concentration range of beta 2-glycoprotein-I. 2. The time dependence of this inhibition is a relatively slow process, which is not fully expressed before 1 h of incubation. 3. The effect of the glycoprotein is not due to a direct interaction with the components of the prothrombinase complex, i.e. factors Xa, Va, Ca2+ or prothrombin, nor is the inhibitory action abolished by increasing concentrations of coagulation factors Xa and Va. This suggests that beta 2-glycoprotein-I causes a reduction of the prothrombinase binding sites of these coagulation factors to platelets or phospholipid vesicles. 4. The prothrombinase activity of platelets stimulated with ionophore A23187 or with collagen plus thrombin is also inhibited by beta 2-glycoprotein-I in a manner similar to that observed for phospholipid vesicles or for lysed platelets. These findings suggest a regulatory role for beta 2-glycoprotein-I in the pathway of blood coagulation.

Blood Platelets↗
PubMed1985

Effect of GSH depletion by 1-chloro-2,4-dinitrobenzene on human platelet aggregation, arachidonic acid oxidative metabolism and cytoskeletal proteins.

Platelet reduced glutathione (GSH) is completely depleted by 1-chloro-2,4-dinitrobenzene (CDNB), which is a substrate for GSH-S-transferase. GSH-depleted platelets: a) aggregate normally at high inducer concentration; b) respond with increased (after arachidonic acid) or depressed (after collagen) aggregability at low inducer concentration; c) show almost no arachidonic acid-induced stimulation of the hexose monophosphate shunt; d) are sensitized to oxidant agents such as diamide, which elicits a faster cytoskeletal protein oxidative polymerization and reversible aggregation. Our results suggest that GSH acts as a reducing cofactor and/or free radical scavenger in the PG-hydroperoxidase step of the cyclooxygenase pathway; moreover, GSH protects membrane and cytoskeletal protein -SH groups from oxidation.

Arachidonic Acid↗
PubMed1985

Platelet-stimulating and membranolytic properties of racemic PAF-acether and analogues.

Three synthetic 1-O-hexadecyl-rac-glycero-3-phosphocholines and the corresponding 1-O-hexadecyl-rac-glycero-3-phosphoric acid methyl esters were tested for platelet-aggregatory and membranolytic properties. Nanomolar concentrations of both the 2-O-acetyl and the 2-O-propionyl derivative of 1-O-hexadecyl-rac-glycero-3-phosphocholine were shown to activate platelets in human platelet-rich plasma. About 625-fold higher concentrations of the corresponding phosphoric acid methyl esters are required to produce the same platelet-stimulating effect. The 1-O-hexadecyl-rac-glycero-3-phosphocholine and the 1-O-hexadecyl-rac-glycero-3-phosphoric acid methyl ester are both nearly ineffective up to final concentrations of 0.1 to 0.2 mM. All tested compounds also cause a graduated lysis of red cells in the range of micromolar concentrations.

Blood Platelets↗
PubMed1985

Localization of phosphatidylethanolamine in the plasma membrane of diamide-treated human blood platelets.

In human blood platelet plasma membranes phosphatidylethanolamine (PE) is asymmetrically distributed between the two leaflets. The main part of this phospholipid is localized at the inner half of the lipid bilayer. Upon stimulation of the cell a substantial transbilayer movement of PE as well as phosphatidylserine occurs and the outer leaflet then provides a procoagulant surface. The thrombin-induced PE flip-flop is inhibited by pretreatment of platelets with diamide, whereas pretreatment of platelets with diamide alone up to 5 mM did not change considerably the localization of PE in the platelet membrane. Thus, cytoskeletal proteins, which are modified by diamide, are not involved in the maintenance of the PE asymmetry but are important for the realization of the agonist-induced events in the platelets.

Adenosine Diphosphate↗
PubMed1984

Chemical modification of cytoskeletal proteins of human blood platelets by diamide.

Incubation of human blood platelets with diamide (azo-dicarboxylic acid-bis-dimethylamide, DIA) influences the aggregation behaviour considerably. Depending on concentration and incubation time DIA induces a reversible aggregation or brings about complete inhibition. The effect is reversible and may be due to the regeneration of reduced glutathione (GSH) which will be oxidized to GSSG by DIA. DIA causes disulfide-linked polymer formation of certain cytoskeletal proteins. At least three polymer families (Pa, Pb, Pc) with different molecular weights are formed depending on dosage and incubation time of DIA. The appearance of a double band in Pa correlates with reversible aggregation, the formation of Pc is always accompanied by a complete inhibition of aggregation. A disturbance of cytoskeleton-membrane interaction by polymer formation can be assumed. GSH serves as a reductant of disulfide-linked polymers whereby a direct link between the maintenance of SH/SS status of platelets and GSH can be established.

Azo Compounds↗
PubMed1984

Influence of diamide on aggregation, cytoskeletal proteins, and arachidonic acid metabolism in human platelets.

Simultaneous addition of diamide (azodicarboxylic acid-bis-dimethylamide, DIA), a SH-oxidizing agent, and collagen causes a deaggregation of otherwise irreversibly aggregating platelets. Thromboxane B2 (TXB2) and 12-HE-TE formation is inhibited depending on the concentration ratio between collagen and DIA. Thus, at 0.25 mM DIA and 20 micrograms/ml collagen neither TXB2 nor 12-HETE were measurable, but a full scale reversible aggregation is induced. Deaggregation is further attained by adding DIA to collagen-induced aggregates at a time, when maximum amplitude has been achieved. Investigation of arachidonic acid (AA) metabolites under these conditions revealed no influence of DIA on AA metabolism. Therefore, AA metabolization seems to play a minor role in collagen-induced aggregation and DIA-induced deaggregation. Polymerization of certain cytoskeletal proteins of the platelets, after addition of DIA, parallels DIA-induced deaggregation. DIA inhibits endogenous AA release, probably by interaction with platelet plasma membrane. DIA seems to inhibit the release of the alpha-granula protein thrombospondin.

12-Hydroxy-5,8,10,14-eicosatetraenoic Acid↗
PubMed1984

[Possible regulatory importance of cellular sulfhydryl groups and reduction metabolic pathways for the activation of human blood platelets].

The activation of the blood platelets is the prerequisite for their participation in physiological and pathological intravasal processes. An aimed influence on the distinct functions of the blood platelets presumes an exact knowledge about course and regulation of the activation of the platelets. Investigations on glucose-6-phosphate-dehydrogenase-deficient platelets and on the effect of glutathione-oxidizing substances on normal platelets showed references to a regulatory significance of the cellular thiol/disulphide state in the process of activation. In this case particularly the arachidonic acid balance and the SH/SS-state of platelet proteins seem to be in close connection with reductive ways of metabolism.

Arachidonic Acids↗
PubMed1984

Cooperative effects of 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine (PAF-acether) and exogenous arachidonic acid in stimulation of human blood platelets.

Cooperative effects of PAF-acether and arachidonic acid in blood platelet activation were studied in human platelet-rich plasma. Using a combination of low concentrations of PAF-acether and subthreshold amounts of exogenous arachidonic acid an enhancement of aggregation and an increased formation of malondialdehyde are obtained. Both effects are completely suppressed either by acetylsalicylic acid or by use of PAF-acether desensitized platelets, indicating that cyclooxygenase products and intact receptors for PAF-acether are of primary importance in the observed synergism. Stimulating activity of PAF-acether on the formation of cyclooxygenase products is also seen with use of higher concentrations of arachidonic acid during mild stimulation of human platelets. This effect of PAF-acether is dependent on the concentration used and is obviously due to changes in the platelet membrane as derived from experiments with lysed platelets. Enhanced capacity of platelets to synthesize endoperoxides and thromboxane in consequence of an in vivo release of PAF-acether into the circulation might be of pathobiochemical relevance in some circumstances.

Arachidonic Acids↗
PubMed1984

Effect of diamide (azodicarboxylic acid-bis-dimethylamide) on arachidonic acid release from human blood platelet phospholipids.

It was found that the thiol-oxidizing agent diamide inhibits the formation of arachidonic acid metabolites in thrombin-stimulated platelets. An inhibition of arachidonic acid release from membrane phospholipids in diamide-treated platelets is concluded since diamide does not diminish the formation of stable metabolites from exogenous arachidonic acid. The role of the cellular thiol-disulfide status for the activities of phospholipases involved in arachidonic acid release is discussed.

Arachidonic Acid↗
PubMed1984

[Proteins and phospholipids of thrombocytes and erythrocytes in glucose-6-phosphate dehydrogenase deficient patients].

Proteins and phospholipids of platelets and red cell membranes of glucose-6-phosphate-dehydrogenase-deficient patients with chronic hemolytic disease were investigated. The enzyme deficiency is not connected with alteration in the protein and phospholipid pattern of platelets. However, marked differences in the diamide-induced protein polymerization between normal and deficient platelets were found. Whereas normal platelets show only a weak polymerization which is reversible during diamide incubation of the cells, the deficient platelets demonstrate a considerable and long lasting protein polymerization. The results point to a direct connection between the thiol-disulfide status of platelets and intracellular level of glutathione, which is able to repair oxidative damages. Two of the six patients showed already protein polymers in their erythrocyte membrane without addition of any SH-oxidizing agent. The phospholipid pattern of the deficient red-cell membranes were comparable to the controls.

Adolescent↗