Enzymatic Reduction of Metmyoglobin in Fish

An enzymatic reducing system for metmyoglobin was studied in two species of fish, bluefin tuna ( Thunnus thynnus ) and jack mackerel ( Trachurus symmetricus ). In both species, the enzyme was primarily located in the midsection of the fish muscle; it had an optimum pH of 7 and an optimum temperature of 25°C. The enzymatic reducing activity of both species was enhanced by anaerobic conditions and partially inhibited by the enzyme catalase, while sodium chloride was not an effective inhibitor. Ascorbic acid as an electron donor and menadione as a mediator were found to be much less efficient than nicotinamide adenine dinucleotide and methylene blue, respectively.