PTERIDINE BIOSYNTHESIS AND NITRIC-OXIDE SYNTHASE IN PHYSARUM-POLYCEPHALUM

Physarum polycephalum, an acellular slime mould, serves as a model sys tem to study cell-cycle-dependent events since nuclear division is nat urally synchronous. This organism was shown to release isoxanthopterin which is structurally related to tetrahydrobiopterin, a cofactor of a romatic amino acid hydroxylases and of nitric oxide synthases (NOSs) ( EC 1.14.13.39). Here, we studied Physarum pteridine biosynthesis in mo re detail and found that high amounts of tetrahydrobiopterin are produ ced and NOS activity is expressed. Physarum pteridine biosynthesis is peculiar in as much as 7,8-dihydroneopterin aldolase (EC 4.1.2.25), an enzyme of folic acid biosynthesis usually not found in organisms prod ucing tetrahydrobiopterin, is detected in parallel. NOS purified from Physarum depends on NADPH, tetrahydrobiopterin and flavins. Enzyme act ivity is independent of exogenous Ca2+ and is inhibited by arginine an alogues. The purified enzyme (with a molecular mass of 130 kDa) contai ns tightly bound tetrahydrobiopterin and flavins. During the synchrono us cell cycle of Physarum, pteridine biosynthesis increases during S-p hase whereas NOS activity peaks during mitosis, drops at telophase and peaks again during early S-phase. Our results characterize Physarum p teridine biosynthesis and NOS and suggest a possible link between NOS activity and mitosis.