Regulation of neuronal nitric oxide synthase by histone,protamine, and myelin basic protein

We examined the effects of endogenous basic proteins rich in the amino acidL-arginine on neuronal NO synthase activity by monitoring cyclic GMP formation in intact neuron-like neuroblastoma N1E-115 cells. Histone, protamine and myelin basic protein significantly stimulated cyclic GMP formation, both in a time- and concentration-dependent manner. These effects were blocked by hemoglobin and NO synthase inhibitors. Removal of the extracellular/intracellular Ca²⁺ gradient by a Ca²⁺ chelator completely abolished the cyclic GMP responses elicited by histone and protamine, suggesting that influex of extracellular Ca²⁺ might be involved in their activation of NO synthase. The effects of myelin basic protein on cyclic GMP formation, however, appeared to be due to Ca²⁺ release from intracellular stores. In cytosolic preparations of rat cerebellum, these basic proteins inhibited the metabolism ofL-arginine intoL-citrulline by NO synthase. We conclude from our findings that endogenous basic proteins might be involved in the regulation of neuronal NO synthase activity. Their effects on the enzyme could be either stimulatory or inhibitory, depending on whether the basic proteins exert their effects extracellularly or intracellularly, respectively.