Guanosine triphosphate cyclohydrolase activity in rat tissues

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Abstract
The GTP cyclohydrolase activity of rat tissues was studied by the measurement of formic acid release and neopterin synthesis from GTP. After gel filtration of a 45% saturated (NH4)2SO4 fraction of liver homogenates, 3 enzyme fractions were separated and named A1, A2 and A3 according to the order of their elution. Fractions A1 and A3 displayed an 8-formyl-GTP deformylase activity; no proof of cyclized product was yet established. This activity was heat-labile and required Mg2+ for maximal activity. Fraction A2 displayed a neopterin-synthetase activity, with dihydroneopterin triphosphate and formic acid formed in stochiometric amounts. Fraction A1* isolated from heat-treated homogenates also produced dihydroneopterin triphosphate. Neopterin synthetase activity in fractions A1* and A2 was heat-resistant and inhibited by Mg2+. In liver the A2 fraction represented 70-75% of the neopterin synthetase capacity and was inhibited by reduced pterines (sepiapterin, dihydrobiopterin and tetrahydrobiopterin) and to a lesser extent by reduced forms of folic acid. In kidney and brain, fraction A1 and A3 GTP 8-formylhydrolase activities were found in significant amounts, in contrast with the neopterin synthetase activity, which was low and appeared to be confined to the A1* fraction.

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