Cyclic Guanosine-3′,5′-Monophosphate and Biopteridine Biosynthesis in Nocardia sp

ABSTRACT Nocardia sp. strain NRRL 5646 contains a nitric oxide synthase (NOS) enzyme system capable of generating nitric oxide (NO) from arginine and arginine-containing peptides. To explain possible roles of the NOS system in this bacterium, guanylate cyclase (GC) and tetrahydrobiopterin (H 4 B) biosynthetic enzymes were identified in cell extracts and in culture media. Cell extracts contained GC activity, as measured by the conversion of GTP to cyclic guanosine-3′,5′-monophosphate (cGMP) at 9.56 pmol of cGMP h −1 mg of protein −1 . Concentrations of extracellular cGMP in culture media were significantly increased, from average control levels of 45 pmol cGMP liter −1 to a maximum of 315 pmol liter −1 , in response to additions of GTP, l -arginine, H 4 B, and sodium nitroprusside to growing Nocardia cultures. On the other hand, the NOS inhibitor N G -nitro- l -arginine and the GC inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one both dramatically decreased extracellular cGMP levels. Activities for GTP-cyclohydrase-1,6-pyruvoyltetrahydropterin synthase and sepiapterin reductase, enzymes essential for H 4 B biosynthesis, were present in Nocardia culture extracts at 77.5 pmol of neopterin and 45.8 pmol of biopterin h −1 mg of protein −1 , respectively. In Nocardia spp., as in mammals, GTP is a key intermediate in H 4 B biosynthesis, and GTP is converted to cGMP by a GC enzyme system that is activated by NO.