Nitric oxide synthase activity of the taste organ of the channel catfish, Ictalurus punctatus

Abstract

The constitutive nitric oxide synthase activity of the catfish taste organ (barbel) was characterized, using the conversion of l-[³H]arginine to l-[³H]citrulline as the index of enzyme activity. The enzyme was dependent on Ca²⁺ (but not calmodulin) and NADPH (but not FAD). Activity was moderately enhanced by tetrahydrobiopterin. Kinetic parameters were K_m = 22 μM and V_max = 25 pmol/min/mg. The enzyme was inhibited by $\text{N}{}^{\mathrm{<mtext>G</mtext>}}\text{-}\text{monomethyl-}\text{l}\text{-arginine}$ (half-maximally at 3 μM) and $\text{N}{}^{\mathrm{<mtext>G</mtext>}}\text{-}\text{nitro-}\text{l}\text{-arginine}$ (half-maximally at 50 μM), and also by sodium nitroprusside and superoxide dismutase. In the presence of millimolar levels of the taste stimulus l-alanine, nitric oxide synthase activity was increased by up to 3-fold, with activation of the enzyme being reversed by $\text{N}{}^{\mathrm{<mtext>G</mtext><mtext>-monomethyl-</mtext><mtext>l</mtext><mtext>-arginine</mtext>}}$. There was no activation of guanylyl cyclase by l-alanine. These data indicate that a constitutive nitric oxide synthase activity is present in the catfish taste organ and that, therefore, nitric oxide may have a role in the biochemical mechanisms underlying taste perception.