MARINE BIOMOLECULES INHIBIT RAT-BRAIN NITRIC-OXIDE SYNTHASE ACTIVITY

A large number of substances of medical importance have been isolated from marine flora and fauna and their chemical structures were elucida ted. Among the many compounds isolated in our laboratories only two co mpounds were identified as neurotoxins as they produced depolarizing e ffects in nerve fibers. The Xestospongin D and Araguspongin C, isolate d and purified to 100% from sponge, Haliclona exigua were tested for t heir effects on rat brain nitric oxide synthase (NOS) activity in vitr o. The results showed that NOS activity was significantly inhibited in a concentration and time dependent manner with an estimated IC50 of 3 1.5 and 46.5 mu M for Xestospongin D and Araguspongin C, respectively, and the maximum inhibition occurred within 3 min of incubation. To ex plore the mechanism of action of these compounds on NOS, we have condu cted kinetic studies with L-arginine, NADPH and Ca2+ in the presence o f IC50 concentrations of these two compounds. The maximum velocity (V- max) and enzyme constant (K-m) were calculated using the Michaelis-Men ten equation. The results show that both compounds are competitive inh ibitors of NOS with the substrate, L-arginine and uncompetitive with N ADPH and free Ca2+. The NOS inhibition by these two compounds was simi lar to N-w-nitro-L-arginine methylester (L-NAME), a known inhibitor of NOS. These results suggest that the marine biomolecules Xestospongin D and Araguspongin C are in vitro modulators of neuronal NOS. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.