TK1 and idiopathic pulmonary arterial hypertension: They also examined the expression of the thymidine metabolism enzymes TK1 and thymidine phosphorylase (TP), as well as a thymidine transporter (equilibrative nucleoside transporter, ENT1), in pulmonary fibroblasts isolated from 9 IPAH patients and 12 donor lungs in culture.