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

Abstract

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 μM. 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 <5 μM. We showed that concentrations of l-NOArg (0.001–3 μM) that did not block the uptake of tritiated arginine ( l-[ 3H]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-[ 3H]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.