GCH1 and septic shock: Biosynthesis of BH4 is catalysed by GTP cyclohydrolase I (GTPCH), a rate limiting enzyme for de novo BH4 biosynthesis, which is encoded by Gch1. Increased circulating plasma biopterins and nitrite/nitrate have been reported in both animals and patients with septic shock [3], [10], [14], [15], [29], [31].