Abstract
Species of the fish genus Carassius survive prolonged anoxia. Nitric oxide (NO) regulates cerebral blood flow in these fish during normoxic conditions whereas adenosine is the main vasoregulating molecule during anoxia. We investigated the calcium ion dependence of Carassius auratus brain NO synthase (NOS) as a function of pH. The physiological pH decrease from 7.2 to 6.8, which takes place during anoxia, greatly decreases NOS activity. This strong pH dependence is mainly due to variation of the calcium sensitivity of the enzyme. The EC 50 is 0.15 μM at pH 7.2 and 2.1 μM at pH 6.8 for the soluble enzyme. The particulate enzyme is also dependent on pH variations. The reduced sensitivity to calcium ions at acidic pH decreases both NO and H 2O 2 production, saving the cells by suppression of the formation of potentially toxic nitrogen and oxygen species. Modulation of NOS activity by variation of its calcium affinity within the range of physiological pH constitutes an important and rapid mechanism to control the formation of NO and H 2O 2 during normoxia–anoxia and anoxia–normoxia transitions.
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