Occurrence of novel types of nitric oxide synthase in the silkworm, Bombyx mori.

Nitric oxide (NO) synthase activity was detected in fat body and the Malpighian tubles of the silkworm, Bombyx mori. Main NO synthase activity in the fat body was Ca(2+)/calmodulin-dependent, inducible by bacterial lipopolysaccharide (LPS) and required NADPH, FAD, FMN, dithiothreitol (DTT) and tetrahydrobiopterin (BH4) as cofactors for the full expression of the activity. The Malpighian tubles contained two types of NO synthase. One was Ca(2+)-independent, calmodulin-dependent and constitutive and the other was Ca(2+)-dependent and constitutive. The former NO synthase required the same cofactors as fat body NO synthase. The activity of Malpighian tuble NO synthases increased dramatically at the end of the last instar period, just prior to spinning. These results indicate that B. mori contains new types of NO synthase, suggesting the wide distribution and different characteristics of this enzyme among vertebrates and invertebrates.