Jürgen Lüthje

The presence of diadenosine 5′,5‴-P1,P3-triphosphate (Ap3A) in human platelets

Diadenosine triphosphate (Ap3A) has been identified and quantified in human platelets using a coupled enzymatic assay specific for Ap3A, after fractionation of acidic extracts with high-performance liquid chromatography. Upon thrombin-induced aggregation, Ap3A is released together with the homologue diadenosine tetraphosphate (Ap4A). Extracts of human platelets do also contain enzymatic activities that degrade diadenosine tetraphosphate as well as diadenosine triphosphate. These enzymes, however, are not released during thrombin-induced aggregation of the platelets.

Diadenosine triphosphate (Ap3A) mediates human platelet aggregation by liberation of ADP

Human platelets store considerable amounts of diadenosine 51,5111-P1, P3-triphosphate, which is released together with the homologue diadenosine tetraphosphate (Ap4A) upon thrombin-induced aggregation (Lüthje,J. & Ogilvie,A. (1983) Biochem. Biophys. Res. Commun. 115, 253-260). We now report that, when added to platelet-rich plasma at 10-20 micron, diadenosine triphosphate gradually induces aggregation. The addition of diadenosine tetraphosphate antagonizes this effect by rapidly disaggregating the platelets. When another physiological but structurally unrelated stimulus, i.e. PAF (Platelet activating factor) is introduced into the system, diadenosine triphosphate drastically enhances and prolongs the aggregatory effect of PAF. Again, Ap4A is antagonistic in this system. The mechanism of Ap3A-stimulation can be explained by the slow and continuous liberation of ADP from Ap3A by the action of a hydrolyzing enzyme which is present in human plasma. Our studies suggest that Ap3A may be physiologically important in providing a relatively long-lived stimulus that can modulate platelet aggregation.

Effects of diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A) on platelet aggregation in unfractionated human blood

SummaryThe effects on platelet aggregation of diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A), both of which are stored in and released from platelet granules, have been studied in unfractionated human blood using a microscopic platelet-count ratio method. Ap3A at submicromolar concentrations induces platelet aggregation whereas the homologue dinucleotide Ap4A has disaggregating potency. In the concentration range between 10−7 to 10−5 M, Ap3A has been found to be as effective as ADP in triggering aggregate formation. These results confirm and essentially extend our recent findings with platelet-rich plasma that Ap3A is able to trigger platelet aggregation by a slow release of ADP from Ap3A which is catalyzed by a plasma hydrolase. Formation of platelet aggregates was also followed kinetically using a turbidometric method which has been developed for this purpose. In contrast to ADP which very rapidly induces a transient state of aggregation, the effect of Ap3A occurs much more slowly but induces the same maximum of aggregation. The duration of the Ap3A stimulus, however, is longer than that of ADP pointing to a potential physiological function of Ap3A as a “masked” source for ADP.

Unproportionally high concentrations of diadenosine triphosphate (Ap3A) and diadenosine tetraphosphate (Ap4A) in heavy platelets

SummaryPlatelets from whole blood were separated into five density subpopulations using a discontinuous Percoll gradient. The content of diadenosine triphosphate (Ap3A), diadenosine tetraphosphate (Ap4A), ADP and ATP were determined in the subfractions. The dinucleotides were directly measured in neutralized, acid-soluble extracts of human platelets with a bioluminescence method not requiring any chromatographic step. When comparing the nucleotide contents of the density subpopulations it became evident that all nucleotides steadily increased with increasing density. Ap3A, Ap4A, ADP and ATP were present in 10-, 7-, 4-and 2-fold higher amounts in the heaviest platelets, respectively, as compared to the subfraction with the lowest density. This finding is practically relevant since the most dense platelet subpopulations may be lost during conventional centrifugation to obtain platelet-rich plasma. Therefore we compared a platelet population obtained from PRP with the platelet population, which had been prepared from whole blood by means of a continuous Percoll gradient. All the four nucleotides investigated were represented in 1.5- to 2-fold higher amounts in the whole blood platelet population. This indicates that PRP does not contain a representative population but lacks part of the large heavy platelets containing the highest amounts of nucleotides.

Extracellular adenine compounds, red blood cells and haemostasis: facts and hypotheses

SummaryPreviously, the role of adenine nucleotides was thought to be confined to the intracellular space of the cell. Research of the last decades has revealed that nucleotides also occur in the extracellular milieu. This survey deals with extracellular adenine compounds in the blood, focussing on their role as chemical mediators in the haemostatic effect of red cells. Erthrocytes may act as pro-aggregatory cells by at least two chemical mechanisms. Firstly, they can enhance platelet aggregation by releasing adenosine diphosphate (ADP), a well known platelet stimulatory substance. ADP is set free when red cells are stressed mechanically, for instance by shear forces generated in the blood stream; ample experimental evidence supporting this view is summarized. Secondly, erythrocytes efficiently take up extracellular adenosine via their nucleoside transporters, thereby removing a potent inhibitor of platelet function. Extracellular adenosine occurs in the blood stream, either directly released from various tissues or as the end product of extracellular adenine nucleotide metabolism, e.g. after degradation of red cell-born ADP or ATP. Finally, a novel mechanism of action of the antithrombotic drug dipyridamole, which has very recently been put forward, is demonstrated. Dipyridamole inhibits platelet function indirectly by blocking the uptake of extracellular adenosine via the nucleoside transporter of red cells; increased adenosine levels in turn are responsible for the antiaggregatory effect of dipyridamole.