Effect of temperature, pH and heme ligands on the reduction of Cygb(Fe3+) by ascorbate

• The reduction of Cygb by ascorbate is examined under different conditions.
• The rate of Cygb reduction increases 6% per °C between 35 °C and 40 °C.
• The rate of Cygb reduction increases ∼3 fold/pH unit between pH6 and pH8.
• Heme Fe3+ and Fe2+ ligands have opposite effect on the Cygb reduction.
• Information provided enables prediction of Cygb-mediated NO metabolism.

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(Fe3+) 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(Fe3+) reduction by ascorbate (Asc) under anaerobic conditions. The standard enthalpy of Cygb(Fe3+) reduction by Asc was determined to be 42.4 ± 3.1 kJ/mol. The rate of Cygb(Fe3+) reduction increased ∼6% per °C when temperature varied from 35 °C to 40 °C. The yield and the rate of Cygb(Fe3+) reduction significantly increases with pH (2–3 times per pH unit), paralleling the formation of the Asc ion (A2−) 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(Fe3+) reduction, but ligands CO and NO allowed the process of Cygb(Fe3+) 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.