Characterization of the Cofactors Involved in Non-enzymatic Metmyoglobin/Methemoglobin Reduction In Vitro

Previous research reported the role of nonenzymatic metmyoglobin (MetMb) and methemoglobin (MetHb) reduction in meat color; however, limited studies have characterized the cofactors involved in nonenzymatic reduction. The objective of this study was to characterize electron donors and carriers in nonenzymatic MetMb and MetHb reduction at various temperatures and postmortem muscle pHs in vitro. Methylene blue and cytochrome c (cyt-c) were evaluated as electron carriers and nicotinamide adenine dinucleotide, reduced form (NADH) and ascorbate were considered as electron donors. All combinations of electron donors and carriers were evaluated in the following order: NADH plus methylene blue, ascorbate plus methylene blue, NADH plus cyt-c, and ascorbate plus cyt-c. Spectrophotometry was utilized to monitor the rates of reduction. The results indicated that methylene blue was an effective electron carrier than cyt-c in the presence of NADH. Temperature and pH had cofactor-specific effects on nonenzymatic MetMb and MetHb reduction. Lower temperature resulted in an increased nonenzymatic MetMb reduction for methylene blue regardless of electron donor (ascorbate, P = 0.03, NADH, P = 0.04). As pH increased, MetHb reduction was enhanced in the presence of ascorbate plus cyt-c. Nonenzymatic MetHb reduction was numerically lower than nonenzymatic MetMb reduction in the presence of NADH plus methylene blue. In summary, in addition to NADH, the current in vitro research demonstrated that ascorbate plus cyt-c could contribute to nonenzymatic MetMb and MetHb reduction at meat-pH and storage temperature.