“Dihydrofolate and other products of the folate and tetrahydrobiopterin pathways did not inhibit … cyclohydrolase I. In addition to their effect on the enzyme activity, Ca2+ and Mg2+ ions catalysed the chemical dephosphorylation of dihydroneopterin triphosphate to non-cyclic dihydroneopterin monophosphate, the substrate for the phosphomonoesterase reaction in folate biosynthesis.”