Role of pH on the calcium ion dependence of the nitric oxide synthase in the carp brain

The role of pH on the calcium dependence of nitric oxide synthase (NOS) of Cyprinus carpio brain was investigated. This fish is known to survive prolonged periods of hypoxia. Under this condition, cerebral blood flow is no longer regulated by nitric oxide (NO). Nitric oxide synthase activity is pH dependent in the range of pH between 7.4 and 6.2 with a decrease when tissue acidifies. At acidic pH, the dependence of the NOS activity on the free Ca2+ concentrations changes considerably and shows an EC50 of 0.13 muM at pH 7.1 and of 5.1 muM at pH 6.2 for the soluble enzyme. The variation in the Ca2+ dependence with acidification is greater for the soluble than for the particulate enzyme. This may be the main factor protecting sudden NO formation mainly during anoxic-normoxic transitions. (C) 2001 Elsevier Science Inc.