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 ba cterium, guanylate cyclase (GC) and tetrahydrobiopterin (H4B) biosynthetic enzymes were identified in cell extracts and in culture media. Cell extract s contained GC activity, as measured by the conversion of GTP to cyclic gua nosine-3',5'-monophosphate (cGMP) at 9.56 pmol of cGMP h(-1) mg of protein( -1). Concentrations of extracellular cGMP in culture media were significant ly increased, from average control levels of 45 pmol cGMP liter(-1) to a ma ximum of 315 pmol liter(-1), in response to additions of GTP, L-arginine, H 4B, and sodium nitroprusside to growing Nocardia cultures. On the other han d, the NOS inhibitor NG-nitro-L arginine and the GC inhibitor 1H-[1,2,4]oxa diazole[4,3-a] quinoxalin-1-one both dramatically decreased extracellular c GMP levels. Activities for GTP-cyclohydrase-1,6-pyruvoyltetrahydropterin sy nthase and sepiapterin reductase, enzymes essential for H4B biosynthesis, w ere 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 H4B biosynthesis, and GTP is c onverted to cGMP by a GC enzyme system that is activated by NO.