The hypervalent muscle pigment ferrylmyoglobin, MbFe(IV)]O, is not reduced by urate monoanions at physiological conditions despite a strong driving force of around 30 kJ mol1 while for low pH, uric acid was found to reduce protonated ferrylmyoglobin, MbFe(IV)]O,H+, efficiently in a bimolecular reaction with k1 ¼ 1.1 0.1 103 L mol1 s1, DH‡ ¼ 66.1 0.1 kJ mol1 and DS‡ ¼ 35.2 0.2 J mol1
K1. For intermediate pH, like for anaerobic muscles and for meat, proton-oupled electron transfer occurs in a transition state, {MbFe(IV)]O/H+/urate}‡, which is concluded to be formed from uric acid and MbFe(IV)]O rather than from urate and MbFe(IV)]O,H+ with k3 ¼ 9.7 0.6 102 L mol1 s1, DH‡ ¼ 59.2 0.1 kJ mol1 and DS‡ ¼ 11.5 0.3 J mol1 K1. The activation parameters as calculated from
the temperature dependence of the pH-reduction profile in aqueous 0.067 mol L1 NaCl (from 25 C up to 40 C), support a mechanism for reduction of hypervalent heme iron, where initial proton transfer to oxo-iron initiates the intermolecular electron transfer from urate to ferrylmyoglobin. The concentration
of the powerful prooxidant ferrylmyoglobin increases strongly during digestion of red meat in the stomach. A concomitant increase in uric acid concentration may serve as an inherent protection against radical formation by ferrylmyoglobin