Metmyoglobin

Metmyoglobin is the oxidized form of the oxygen-carrying hemeprotein myoglobin.

Metmyoglobin is the cause of the characteristic brown colouration of meat that occurs as it ages.

In living muscle, the concentration of metmyoglobin is vanishingly small, due to the presence of the enzyme metmyoglobin reductase, which, in the presence of the cofactor NADH and the coenzyme cytochrome b4 converts the Fe3+ in the heme prosthetic group of metmyoglobin back to the Fe2+ of normal myoglobin.

Metmyoglobin after being oxidized by myoglobin shows the undesirable brown color which can be seen in many types of meat.

[2] The metmyoglobin reducing activity varies across species and was studied particularly in beef, porcine, bison, deer, emu, equine, goats and sheep.