Characterisation of nitric oxide synthase activity in the tropical sea anemone Aiptasia pallida

The presence of nitric oxide synthase (EC 1.14.23 NOS) activity is demonstr ated in the tropical marine cnidarian Aiptasia pallida (Verrill). Enzyme ac tivity was assayed by measuring the conversion of [H-3]arginine to [H-3]cit rulline. Optimal NOS activity was found to require NADPH. Activity was inhi bited by the competitive NOS inhibitor N-G-methyl-L-arginine (L-NMA), but n ot the arginase inhibitors L-valine and L-ornithine. NOS activity was predo minantly cytosolic, and was characterised by a K-m for arginine of 19.05 mu M and a V-max of 2.96 pmol/min per mu g protein. Histochemical localisatio n of NOS activity using NADPH diaphorase staining showed the enzyme to be p redominantly present in the epidermal cells and at the extremities of the m esoglea. These results provide a preliminary biochemical characterisation a nd histochemical localisation of NOS activity in A. pallida, an ecologicall y important sentinel species in tropical marine ecosystems. (C) 2000 Elsevi er Science Inc. All rights reserved.