Selective nitration of prostacyclin synthase and defective vasorelaxation in atherosclerotic bovine coronary arteries.

Prostacyclin synthase (PCS) is an enzyme with antithrombotic, antiproliferative, and dilatory functions in the normal vasculature, and inactivation of PCS by tyrosine nitration may favor atherosclerotic processes. Here, we show that PCS is nitrated and inactivated in early stage atherosclerotic lesions (focal intimal thickenings). Immunoprecipitation with antibodies raised against nitrotyrosine yielded PCS as the main nitrated protein in blood vessels. Moreover, we identified two nitrated degradation products of PCS with molecular mass of 30 and 46 kd, which were selective for atherosclerotic tissue. Agonist (acetylcholine, angiotensin II)-induced prostacyclin formation was decreased in atherosclerotic vessels compared with normal tissue, whereas PGE2 formation was increased and cyclooxygenase activity remained unchanged. A selective loss of PCS activity was confirmed by direct measurement of enzymatic activity. In line with this, we observed defective relaxation of early atherosclerotic vessels following vasoconstrictive stimulation. This functional impairment was completely reversed by coincubation with an antagonist of the thromboxane/PGH2 receptor but not by a thromboxane synthase inhibitor. These data suggest that reduced PCS activity in atherosclerotic arteries prevents the rapid use of PGH2, which accumulates and acts as an agonist on the vasoconstrictive thromboxane receptor.