Kinetic Analysis of the Thermal Inactivation Behavior of AMP Deaminase and IMPase in Each Muscle Type of Yellowtail Seriola quinqueradiata

In this study, a kinetic analysis was conducted to clarify the thermal inactivation behavior of AMP deaminase and IMPase, enzymes involved in the generation and degradation of inosine 5′-monophosphate (IMP) in the dorsal ordinary muscle (OM) and dark muscle (DM) of yellowtail Seriola quinqueradiata. Both enzymes were extracted from each part of the fish muscle, heated in the range of 50–60 °C, and then measured for residual enzyme activity. Based on these data, kinetic analysis was performed. When comparing fish muscle types, the thermal stability at 50 °C and 55 °C and the temperature dependence of both AMP deaminase and IMPase tended to be higher in the DM. When comparing the two enzymes, the thermal stability of IMPase was higher than that of AMP deaminase at 50 °C in both muscle types. These results suggest that to prepare heated yellowtail muscle with a high IMP content, it is important to consider the thermal inactivation behavior of enzymes and use slow heating to maintain AMP deaminase activity and produce sufficient IMP in OM. For DM, rapidly increasing the product temperature to ≥60 °C to inactivate IMPase is required to preserve the IMP content.