Update on Endogenous HNO Production: An Exploration of Unanswered Questions and Challenges

Abstract

This review consolidates the evidence supporting endogenous HNO generation through enzymatic and non‐enzymatic pathways, emphasizing advancements in real‐time HNO sensing within living systems. Azanone (nitroxyl, HNO) is the one‐electron‐reduced congener of nitric oxide (NO•) and shares various biological actions. HNO exhibits unique properties, including positive inotropic and lusitropic effects, along with resistance to scavenging by reactive oxygen species (ROS), particularly superoxide. Research on HNO has intensified over the last two decades, focusing on its reactivity and the prospect of endogenous formation due to its potential significance. The high reactivity of HNO arises from reactions with biologically relevant species such as oxygen, NO•, and thiols, as well as self‐dimerization. Detecting and quantifying HNO has posed challenges, initially relying on nitrous oxide (N2O) detection, a byproduct of dimerization. Recent advancements, including fluorescent probes and a specific electrochemical nanomolar‐level sensor, have facilitated direct detection, enhancing understanding of HNO reactivity and formation.