Finding. Cytochrome c oxidase can make nitric oxide from nitrite, and adenine nucleotides decide whether that activity survives oxygen. In solubilized yeast and mouse-brain enzyme, driven hard with ascorbate/TMPD/cytochrome c and 1 mM nitrite, ADP keeps NO coming across the entire oxygen range tested, up to 175 µM. ATP’s effect depends on the isoform: a little inhibition on Va, strong stimulation on Vb in anoxia. After a hypoxic shift, cellular ADP/ATP and later-measured Cco/NO both bump. The authors say this is gating of catalytic capacity. It is not a proof that Cco makes a meaningful share of cellular NO at physiological nitrite in coupled mitochondria.
Why this paper matters
Cco/NO is a recurring hypoxic-signaling hypothesis that often dies in the “but oxygen” and “but other NO sources” objections. This paper meets both with numbers and with an explicit non-claim. Isoform-dependent ATP gating is new leverage: the same nucleotide is not one knob.
For a mitochondrial desk, Complex IV as a regulated nitrite reductase is the object. The humility paragraph is why the brief stays trustworthy.
What they actually measured
Solubilized Cco from yeast and mouse brain; nitrite; donor system; ADP/ATP versus oxygen; Va/Vb split; aa3-normalized multi-turnover rates; hypoxic-shift nucleotides and follow-up activity.
How to read the score
High seventies. Careful enzymology, isoform logic, self-limited interpretation. Confidence is high for the assay, low for in vivo NO budgets. Score 79.
What to do with it
If you model hypoxic NO, Cco isoforms, or nucleotide gating of Complex IV, pull the oxygen-range curves and the Va/Vb ATP split. Do not write that mitochondria are a major NO source from this brief. The directional implication is that metabolic state and Cco isoform composition can gate nitrite-to-NO capacity even in surprisingly high oxygen, at least in this assay.