Partial Purification and Properties of Guanosine Triphosphate Cyclohydrolase, the First Enzyme in Pteridine Biosynthesis, from Comamonas sp. (ATCC 11299a)

GTP cyclohydrolase, the first enzyme in pteridine biosynthesis, has been purified from cell-free extracts of Comamonas sp. (ATCC 11299a). A 20-fold purification was obtained using ammonium sulfate fractionation, adsorption on calcium phosphate gel, and DEAE-Sephadex chromatography. Because of extreme instability, less than 2% of the total activity was generally recovered. The enzyme can be assayed by its ability to produce formic acid from C-8 of GTP. This activity requires Mg++, and the magnesium requirement is quite specific. The activity is markedly increased by preliminary incubation of the enzyme with sulfhydryl compounds, and this activation is prevented by GTP, but not by Mg++. No inhibitors or effectors have been observed. The major pteridine product has absorption and fluorescence properties similar to those of neopterin. The presence of phosphate has been shown by the use of GT32P as substrate. The product at this stage of purification appears to be a cyclic phosphate of neopterin (2-amino-4-hydroxy-6-trihydroxypropyl pteridine). Chemical and enzymatic evidence for this structure has been obtained. The properties and products of this enzyme are different than those of a previously described enzyme of similar function from E. coli. The possible reasons for the production of a cyclic phosphate by Comamonas are discussed.