*2.7. Oxygen Release Efficacy in Erythrocytes*

High-resolution respirometry, an Oroboros Oxygraph-2 K (Oroboros Instruments, Innsbruck, Austria), was used to measure the O2 pressure (mmHg) and O2 flux per volume (pmol·s−1·mL−1) of erythrocytes at 0-, 1-, and 4 mM lactate during hypoxia (PO2 = 20 ± 3 mmHg) and normoxia (PO2 = 147 ± 3 mmHg) in HBSS medium, respectively. Hypoxic conditions were prepared by gassing with nitrogen (N2) gas. After heating at 37 ◦C and equilibration and calibration for the target PO2 conditions, 2 × <sup>10</sup><sup>6</sup> isolated erythrocytes were added, and after the signaling stabilized, 2 and 6 μL of 1 M lactic acid (Sigma) were sequentially added to form 1 and 4 mM lactic acid environments to simulate the rest and lactate threshold conditions, respectively (Figure S1). All samples were analyzed in triplicate, and the intraassay CV was 3.9 ± 0.8%.
