Amino acids differentially inhibit the l-[3H]arginine transport and nitric oxide synthase in rat brain synaptosomes

The nitric oxide synthase (NOS) present in the cytosol obtained from rat cerebral cortex synaptosomes was inhibited by N G -nitro- l -arginine ( l -NOArg) with an IC 50 value of ∼0.06 μ M. This compound did not affect the transport of l -arginine ( l -Arg) into synaptosomes at concentrations up to 100 μM but other potential inhibitors of NOS (N G -monomethyl- l -arginine, N G -amino- l -arginine and l -arginine methyl ester) inhibited l -Arg transport at a concentration μ M. We showed that concentrations of l -NOArg (0.001–3 μM) that did not block the uptake of tritiated arginine ( l -[ 3 H]Arg) inhibited the catalytic activity of NOS in intact synaptosomes. l -NOArg at a concentration of 1 μM inhibited the cytosolic enzyme by 98.0 ± 2.0% of the total NOS activity whereas the enzyme studied in the intact synaptosomes was only inhibited by 75 ± 5% which suggested that the NOS in synaptosomes is not fully accessible to the external l -NOArg. On the other hand, l -Lysine did not inhibit the cytosolic NOS activity of ruptured synaptosomes but at a concentration that blocked 50.0 ± 4.5% of l -[ 3 H]Arg uptake it inhibited the NOS activity in intact synaptosomes by 12.6 ± 3.6%, suggesting that the transport of l -Arg may be an important regulatory step in the pathway for nitric oxide generation.