Exogenous ligand effects on S-nitrosohemoglobin formation in reactions of methemoglobin with nitric oxide

Abstract

Blood's function of carrying oxygen to tissues is regulated by physiological oxygen gradients that are coupled to vasoconstriction and vasodilation by nitric oxide bioactivity. However, the mechanism by which local oxygen tension utilizes nitric oxide signaling in order to regulate blood flow remains a major unanswered question in biology. Hemoglobin in red blood cells appears to be an ideal sensor, but lack of knowledge about hemoglobin's chemistry with nitric oxide creates a problem for understanding how hemoglobin induces vasodilation. A central focus of this work was to illuminate complexity and response of nitric oxide interactions with hemoglobin while showing how S-nitrosohemoglobin plays a pivotal role in this response. Electronic absorption and electron paramagnetic resonance spectroscopy were employed to observe reactions of low-spin methemoglobin species with nitric oxide. Models were developed and tested to establish the chemistry involved and the global hierarchy of reactions. De minimis models for all experiments are reported. Electron paramagnetic resonance spectra confirmed the formation of low-spin iron species that, upon the addition of nitric oxide, showed a decrease in low-spin methemoglobin species and the formation of nitrosyl hemoblobin with a beta-subunit preference. We report the yield of S-nitrosohemoglobin production in reactions of nitric oxide with various methemoglobins distinguished by axial ligand and iron spin state.