Effect of bitterns and constituent salts on the inosinate phosphatase activity in horse mackerel muscle

Japanese horse mackerel (Trachurus japonicus) is a widely consumed commercial fish in Japan. Inosinic acid is one of the taste components generated by the degradation of ATP in fish muscle. In this study, we investigated the efficacy of bitterns for the preservation of fish taste by monitoring inhibition of inosinate phosphatase (IMPase) activity in horse mackerel muscles. The effects of two different kinds of bitterns produced from seawater (SW) or the ion exchange membrane (IEM) method were used. The relative activity of inosinate phosphatase was measured at different pH values and at different bitterns concentrations. We found that both SW and IEM bitterns significantly inhibited the enzyme when used at concentrations of 8% and 17%, respectively; interestingly, however, with the use of 1.7% bitterns, there was a profound increase in the activity of inosinate phosphatase. To further understand this phenomenon, the effects of different salts commonly present in bitterns were also investigated. We found that each salt (NaCl, MgCl2, MgSO4 and CaCl2) had significantly different pH dependences and different effects on IMPase activity. MgCl2 and MgSO4 were found to be major constituents of bitterns that affected IMPase activity. These results indicated that with the proper concentration and pH, bitterns could effectively inhibit IMPase activity and therefore have the potential to be used as effective additives for the preservation of fish taste.