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

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.