Effect of temperature, pH and heme ligands on the reduction of Cygb(Fe(3+)) by ascorbate.

Cytoglobin (Cygb) plays a role in regulating vasodilation in response to changes in local oxygen concentration by altering the rate of nitric oxide (NO) metabolism. Because the reduction of Cygb(Fe(3+)) by a reductant is the control step for Cygb-mediated NO metabolism, we examined the effects of temperature, pH, and heme ligands on the Cygb(Fe(3+)) reduction by ascorbate (Asc) under anaerobic conditions. The standard enthalpy of Cygb(Fe(3+)) reduction by Asc was determined to be 42.4 ± 3.1 kJ/mol. The rate of Cygb(Fe(3+)) reduction increased ~6% per °C when temperature varied from 35°C to 40°C. The yield and the rate of Cygb(Fe(3+)) reduction significantly increases with pH (2-3 times per pH unit), paralleling the formation of the Asc ion (A(2-)) and the increased stability of reduced state of heme iron at high pH values. Heme ligand cyanide (CN(-)) decreased the yield and the rate of Cygb(Fe(3+)) reduction, but ligands CO and NO allowed the process of Cygb(Fe(3+)) reduction to continue to completion. Critical information is provided for modeling and prediction of the process of Cygb-mediated NO metabolism in vessels in a range of temperature and pH values.