The Effect of Heat and Various Salts on the Activity of the Inosinate Monophosphate-degrading Enzyme in Horse Mackerel

Introduction: Horse mackerel is a popular fish consumed worldwide and is often salted or dried for long-term storage. Maintenance of the quality of stored fish necessitates the suppression of inosinate monophosphate (IMP) degradation. Since IMP is degraded by inosinate phosphatase (IMPase), the preservation technology of mackerel can be improved by elucidating the property of IMPase.

Purpose: This study aimed to clarify the changes in IMPase activity in horse mackerel under varying heat, salt, and pH conditions.

Methods: Fish flesh homogenate was dialyzed against water for 2.5 days, and the dialysate was filtered and diluted twice at 10°C (enzyme solution). The standard reaction mixture included buffer (0.2M succinic acid/NaOH pH 4-6 and 0.2M maleic acid/0.2M Tris/NaOH pH 6-8), 25 mM IMP, 10% of various salts (NaCl, MgCl₂, CaCl₂, or MgSO₄) or water, and enzyme solution. The reaction mixture was incubated overnight after heating for 2 h at 45°C or without the heat treatment. The reaction was stopped using 10% perchloric acid, and the amount of inorganic phosphate liberated in the supernatant was determined.

Results: IMPase activity decreased by heat treatment and in the presence of NaCl, MgCl₂, and CaCl₂ at pH 7-8. The enzyme that retained high activity at high pH could not withstand heat or salt treatments. IMPase activity was unaffected by heat and salt treatments at pH 5. However, the enzyme that showed high activity at low pH could withstand heat or salt treatments. The IMPase activity decreased at low pH and increased at high pH in the presence of MgSO₄ regardless of whether heat treatment was applied or not.

Significance: These data suggest that the quality of horse mackerel could be maintained during the heating process by the addition of NaCl, MgCl₂, and CaCl₂ at high pH or by the addition of MgSO₄ at low pH.