THE NITRIC-OXIDE TRANSDUCTION PATHWAY IN TRYPANOSOMA-CRUZI

A nitric oxide synthase was partially purified from soluble extracts o f Trypanosoma cruzi epimastigote forms. The conversion of L-arginine t o citrulline by this enzyme activity required NADPH and was blocked by EGTA. The reaction was activated by Ca2+, calmodulin, tetrahydrobiopt erin, and FAD, and inhibited by N-omega-methyl-L-arginine. L-Glutamate and N-methyl-D-aspartate stimulated in vivo conversion of L-arginine to citrulline by epimastigote cells. These stimulations could be block ed by EGTA, MK-801, and ketamine and enhanced by glycine. A sodium nit roprusside-activated guanylyl cyclase activity was detected in cell-fr ee, soluble preparations of T. cruzi epimastigotes. L-Glutamate, N-met hyl-D-aspartate, and sodium nitroprusside increased epimastigote cycli c GMP levels. MK-801 bound specifically to T. cruzi epimastigote cells . This binding was competed by ketamine and enhanced by glycine or L-s erine. Evidence thus indicates that in T. cruzi epimastigotes, L-gluta mate controls cyclic GMP levels through a pathway mediated by nitric o xide.