Metmyoglobin reductase activity in porcine m. longissimus dorsi muscle

Reduction of metmyoglobin by metmyoglobin reductase extracted from porcine m. longissimus dorsi was found to depend on the presence of NADH and to a lesser degree on the presence of an electron transfer mediator (ferrocyanide). The porcine metmyoglobin reductase was found to reduce equine metmyoglobin and metmyoglobin isolated from pigs hearts using a method described for bovine metmyoglobin. A linear increase in the rate of reduction as a function of amount of enzyme extract was observed, while the rate as a function of NADH concentration increased to a saturation level. In the pH range 6.0-7.1, the rate of reduction of porcine metmyoglobin by porcine metmyoglobin reductase increased at decreasing assay pH, apparently approaching a maximal rate around pH 6. From the temperature dependence of the maximal rate of the enzyme catalyzed reduction of porcine metmyoglobin an apparent activation energy of 33 kJ mol(-1) was calculated. Porcine metmyoglobin was also reduced by NADH in a non-enzymatic reaction, with a rather similar activation energy showing (i) that porcine metmyoglobin is more closely associated with the reductase complex than bovine metmyoglobin and not so easily separated, or (ii) that porcine metmyoglobin reduction is more facile as a non-enzymatic reaction and less dependent on electron transfer mediators compared to bovine metmyoglobin reduction.