| Summary: | Metabolic modulation of macrophage activation has emerged as a promising strategy lately in immunotherapeutics. However, macrophages have a broad spectrum of functions and thus, understanding the exact metabolic changes that drive a particular immune response, is of major importance. In our previous work, we have reported a key role of nitric oxide (NO<sup>●</sup>) in two(2)-signal activated macrophages [M(2-signals)]. Further characterization using metabolic analysis in intact cells, showed that the basal and maximal respiration levels of M(2-signals) were comparable, with cells being unresponsive to the injections-inducd mitochondrial stress. Here, we show that excessive NO<sup>●</sup> secretion by the M(2-signals) macrophages, interferes with the oxygen (O<sub>2</sub>) consumption measurements on cells using the seahorse metabolic analyzer. This is attributed mainly to the consumption of ambient oxygen by NO<sup>●</sup> to form NO<sub>2</sub><sup>−</sup> and/or NO<sub>3</sub><sup>−</sup> but also to the reduction of O<sub>2</sub> to superoxide anion (O<sub>2</sub><sup>●−</sup>) by stray electrons from the electron transport chain, leading to the formation of peroxynitrite (ONOO<sup>−</sup>). We found that reactive species-donors in the absence of cells, produce comparable oxygen consumption rates (OCR) with M(2-signals) macrophages. Furthermore, inhibition of NO<sup>●</sup> production, partly recovered the respiration of activated macrophages, while external addition of NO<sup>●</sup> in non-activated macrophages downregulated their OCR levels. Our findings are crucial for the accurate metabolic characterization of cells, especially in cases where reactive nitrogen or oxygen species are produced in excess.
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