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

Abstract

The presence of nitric oxide synthase (EC 1.14.23 NOS) activity is demonstrated in the tropical marine cnidarian Aiptasia pallida (Verrill). Enzyme activity was assayed by measuring the conversion of [ 3H]arginine to [ 3H]citrulline. Optimal NOS activity was found to require NADPH. Activity was inhibited by the competitive NOS inhibitor N G-methyl- l-arginine ( l-NMA), but not the arginase inhibitors l-valine and l-ornithine. NOS activity was predominantly cytosolic, and was characterised by a K m for arginine of 19.05 μM and a V max of 2.96 pmol/min per μg protein. Histochemical localisation of NOS activity using NADPH diaphorase staining showed the enzyme to be predominantly present in the epidermal cells and at the extremities of the mesoglea. These results provide a preliminary biochemical characterisation and histochemical localisation of NOS activity in A. pallida, an ecologically important sentinel species in tropical marine ecosystems.