SUCO and juvenile idiopathic arthritis: The above described findings in JIA patients can be explained as follows: Levels of BH4 drop despite the inflammation-induced increase in GTP–CH1, which is the rate-limiting enzyme for the biosynthesis of BH4 from guanosine triphosphate (GTP), because NH2P3 is largely used for neopterin synthesis at the expense of the synthesis of BH4 [31,39].