Nitric oxide synthase activity in molluscan hemocytes

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Abstract
The hemocytes of the freshwater snail Viviparus ater have nitric oxide synthase (NOS) activity, as demonstrated by [3H]citrulline and nitrite+nitrate formation. The enzyme is NADPH dependent and is competitively inhibited by the mammalian NOS inhibitor (K i = 4.7 μM). The K m for l‐arginine is 2.5 μM. 70% of the total activity is observed at very low free Ca2+ concentration (3 nM). LPS treatment increased total NOS activity 2.4 fold. The activity is partly present in the non‐soluble fraction of hemocytes (24% and 8% in non‐stimulated and LPS‐stimulated snails, respectively). An antiserum to the C‐terminal synthetic pentadecapeptide of the rat cerebellar NOS inhibited the enzyme activity in a concentration‐dependent manner. This is the first biochemical demonstration of the existance of NOS activity in molluscan hemocytes, the cells responsible for defence mechanisms.