Hydrogen sulphide pathway contributes to the enhanced human platelet aggregation in hyperhomocysteinemia

Homocysteine is metabolized to methionine by the action of 5,10methylenetetrahydrofolate reductase (MTHFR). Alternatively, by thetransulfuration pathway, homocysteine is transformed to hydrogen sulphide (H2S),through multiple steps involving cystathionine beta-synthase and cystathioninegamma-lyase. Here we have evaluated the involvement of H2S in the thromboticevents associated with hyperhomocysteinemia. To this purpose we have usedplatelets harvested from healthy volunteers or patients newly diagnosed withhyperhomocysteinemia with a C677T polymorphism of the MTHFR gene (MTHFR++). NaHS(0.1-100 microM) or l-cysteine (0.1-100 microM) significantly increased plateletaggregation harvested from healthy volunteers induced by thrombin receptoractivator peptide-6 amide (2 microM) in a concentration-dependent manner. Thisincrease was significantly potentiated in platelets harvested from MTHFR++carriers, and it was reversed by the inhibition of either cystathioninebeta-synthase or cystathionine gamma-lyase. Similarly, in MTHFR++ carriers, thecontent of H2S was significantly higher in either platelets or plasma comparedwith healthy volunteers. Interestingly, thromboxane A2 production was markedlyincreased in response to both NaHS or l-cysteine in platelets of healthyvolunteers. The inhibition of phospholipase A2, cyclooxygenase, or blockade ofthe thromboxane receptor markedly reduced the effects of H2S. Finally,phosphorylated-phospholipase A2 expression was significantly higher in MTHFR++carriers compared with healthy volunteers. In conclusion, the H2S pathway isinvolved in the prothrombotic events occurring in hyperhomocysteinemic patients.