Response to Letter Regarding Article “Improvement in Outcomes After Cardiac Arrest and Resuscitation by Inhibition of S-Nitrosoglutathione Reductase”

We thank Tanus-Santos and colleagues for their interest and comments on our recent article that showed protective effects of GSNOR inhibition or deletion on outcomes after cardiac arrest.¹ They argued that oral administration of nitrite or nitrate are more readily available and may offer advantages over GSNOR inhibitors in the therapy of disease conditions such as hypertension. Their opinion is based on their own experimental observation (oral administration of nitrite or nitrate increase circulating levels of S-nitrothiols and tissue nitrosylation),² and a speculation (oral nitrite or nitrate enhances S-nitrothiols formation which may inhibit GSNOR and lead to accumulation of tissue GSNO). Although we agree that oral nitrite or nitrate treatment may have potential benefits in the therapy of some cardiovascular diseases, there are important differences between administration of nitrite (or any other NO donor compounds) and GSNOR inhibition especially after cardiac arrest (CA). In our study, GSNOR activity was increased in plasma of patients resuscitated from out-of-hospital CA (OHCA) and in multiple organs and plasma of mice resuscitated from experimental CA. Increased GSNOR activity was associated with decreased number of S-nitrosylated proteins in the brain, but not in the liver, of post-CA mice. Pharmacological inhibition or deletion of GSNOR restored number of S-nitrosylated proteins in the brain after CA, without affecting levels of S-nitrosylation in the liver. Interestingly, GSNOR deletion or inhibition selectively preserved the S-nitrosylation levels of proteins concentrated in two broad categories; mitochondrial energy homeostasis and membrane trafficking. These observations indicate that GSNOR controls S-nitrosylation levels of “selected” proteins in some, but not all, tissues after CA, presumably due to subcellular localization of the enzyme in certain tissues. On the other hand, administration of nitrite or nitrate will increase S-nitrosylation levels of proteins in all tissues in an undiscriminating manner. It is well established that aberrant and uncontrolled increase of S-nitrosylation levels adversely affects cellular function.^{3, 4}

Whether inhibition of GSNOR by S-nitrosylation inhibits GSNOR activity after CA remains to be determined. We have recently reported that plasma NO consumption is increased in post-OHCA patients and in mice resuscitated from experimental CA.⁵ The increased plasma NO consumption was associated with hemolysis-induced release of oxyhemoglobin, whereas plasma NO consumption was attenuated by NO inhalation. If GSNOR is inhibited by S-nitrosylation, it is conceivable that depletion of NO bioactivity after CPR disinhibited GSNOR thus further decreasing NO bioavailability after CA. Elucidation of the mechanisms regulating GSNOR activity and NO consumption may enable exploitation of this enzyme system to precisely control NO/S-nitrosylation signaling in a personalized manner not only after cardiac arrest but also in other cardiovascular diseases.