Thermal stabilities of inosine monophosphate-degrading enzymes in several fish muscles

The compositions and inosinic acid contents of ATP-related compounds in various processed fish meat products were examined in this study. Moreover, the relationships between these variables and the heating processes used on the products were investigated. Dried and salted fish meat products manufactured immediately after manufacturing, were found to maintain some inosinic acid contents, whereas those stored for a long period of time had low inosinic acid contents. It was observed that the inosine content in processed fish meat products manufactured without the heat treatment was decreased during the storage at 5°C. Therefore, the low inosinic acid content in such processed fish meat products was considered to be because the maintenance of inosine monophosphate (IMP)-degrading enzyme activity, resulting in the decomposition of inosinic acid even under refrigerated storage. The inosine content of unheated water extract decreased faster than that of fish meat product, and the effect of heating temperature could be observed more clearly. The results suggested that IMP-degrading enzymes in club mackerel, Atlantic bluefin tuna, and Japanese jack mackerel were deactivated by heating at 70°C or higher, while those in fine patterned puffer and half-smooth golden pufferfish were deactivated with heating at 60°C or higher. Since IMP decomposition was suppressed incrementally with heating, it appears that the multiple enzymes involved in the decomposition of inosinic acid might have varying thermal stabilities.