NITRIC-OXIDE SYNTHASE ACTIVITY OF THE TASTE ORGAN OF THE CHANNEL CATFISH, ICTALURUS-PUNCTATUS

The constitutive nitric oxide synthase activity of the catfish taste organ (barbel) was characterized, using the conversion of L-[H-3]arginine to L-[H-3]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 mu M and V-max = 25 pmol/min/mg. The enzyme was inhibited by N-G-monomethyl-L-arginine (half-maximally at 3 mu M) and N-G-nitro-L-arginine (half-maximally at 50 mu M), 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 N-G-monomethyl-L-arginine. 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.