Nitric oxide synthase activity in tissues of the bovine eye

Background: Nitric oxide synthase (NOS) is present in many ocular tissues where it may have different physiological functions. This warrants a thorough characterization of NOS activity in the eye. Methods: NOS distribution and its biochemical properties were determined in the retina, choroid, ciliary processes (CP), and trabecular meshwork (TM), Results: Retinal NOS required NADPH (diphenylene-iodonium, a flavoprotein inhibitor, which inhibited enzyme activity with an IC50 of 0.36 mu M, FAD (40 mu M), FMN (40 mu M), and BH4 (4 mu M) as cofactors for optimal activity. Ocular NOS appeared to be regulated by free divalent cations, since its activity was inhibited by EDTA (slopes >3.0 and IC50 values of 12.8, 19.7, and 53 mu M, respectively). Ocular NOS required calmodulin, since NOS activity was inhibited by trifluoperazine (calmodulin inhibitor, IC50=41 mu M). NOS activity is widely distributed in the eye, (choroid >reinta >CP >TM) and is mainly cytosolic (70-95%). L-Arginine analogs inhibited NOS in the retina, choroid, and TM. In all three tissues, N-G-methyl-L-arginine displayed the highest affinity for inhibition (IC50=0.2-0.7 mu M) followed by canavanine (IC50=13-33 mu M), while aminoguanidine only weakly inhibited NOS (IC50=93-179 mu M). Conclusion: In all tissues, the order of potency of inhibition points to the presence of constitutive rather than inducible NOS. Moreover, it is possible that TM contains more than a single form of NOS.