Keywords: amino-BH4; 4-amino-(6R)-5,6,7,8-tetrahydro-l-biopterin; BAECs; bovine aortic endothelial cells; BH2; 7,8-dihydro-l-biopterin; BH4; (6R)-5,6,7,8-tetrahydro-l-biopterin; cGMP; 3â²,5â²-cyclic guanosine monophosphate; DAHP; 2,4-diamino-6-hydroxypyrimidine;