Purification and Properties of Guanosine Triphosphate Cyclohydrolase and a Stimulating Protein from Comamonas Sp. (ATCC 11299a)

GTP cyclohydrolase and a stimulating protein were purified from extracts of Comomonas sp.(ATCC 11299a).The enzyme, designated Fraction A, catalyzed the production of formic acid and a dihydropteridine (PLOWNAN, J., CONE, J. E., AND GUROFF, G. (19'74) J. Biol.Chem.249, 5559~5564) from GTP.No cofactors or metals were required for the reaction.Fraction A can exist in two molecular weight forms demonstrated during sucrose density gradient centrifugation at low (5 mM 'Iris) and high (50 mM Tris) ionic strength.The stimulating protein, designated Fraction B, stimulated the activity of Fraction A at high ionic strength.Magnesium ion was required for this stimulation.Highly purified GTP cyclohydrolase was obtained after affinity chromatography of Fraction A. The activity of the purified enzyme was influenced by: (a) the amount of enzyme protein present; (b) the ionic strength of the assay medium; and (c) the presence of the stimulatory protein plus MgC12.The above variables were shown to affect the basic kinetic values, K,,, and V,,,, of the reaction.Pteridine production in Comamonas sp.1 depends on the enzyme GTP cyclohydrolase: which catalyzes the conversion of GTP to formic acid and B-substituted pterins (1).We have previously reported on the partial purification of this enzyme and on an unusual pteridine product thought to be a cyclic phosphate derivative of neopterin (2).However, a detailed examination of the reaction properties and unequivocal identification of the pteridine product were prevented by the apparent lability of the enzyme during purification, and by the limited amount, of product obtained from incubation mixtures.Similar enzymes, involved in the synthesis of the pteridine por-1 The Tenth Edition of the American Type Culture Collection Catalog lists the organism as a Pseudomonks sp.Although originallv classified as a Pseudomonas SD.. the Eighth Edition of the Catkogue listed the organism as acornanon& sp.* We have adopted the nomenclature of Burg and Brown (1968) who described the purification and properties of the enzyme from Escherichia coli.