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.