Seabass muscle proteins : Physicochemical properties, degradation and denaturation
โปรตีนกล้ามเนื้อของปลากะพงขาว : สมบัติทางเคมี-ฟิสิกส์ การย่อยสลายและการสูญเสีย
สภาพ
Soottawat Benjakul, Asst. Prof., D. of Food Tech., F. of Agro-Industry, PSU.
Suganya Chanthachum, D. of Food Tech., F. of Agro-Industry, PSU.
Corresponding e-mail : bsoottaw@ratree.psu.ac.th
Grant : Government Budget
Published : Research Report
Key words : seabass muscle proteins, degradation, denaturation
Compositions and properties of seabass (Lates calcarifer) muscle proteins were studied. The dorsal and ventral muscle had the similar nitrogenous compositions and amino acid profile. Base on
the inactivation rate constant (KD) of Ca2+-ATPase, seabass actomyosin was stable at 0-20oC and pH 6.5-7.5.
Physicochemical changes of free and intact actomyosin from seabass muscle during 10 days
of iced storage were investigated in the absence and presence of various chemical reagents. The sulfhydryl content in actomyosin solution decreased significantly (p<0.05) throughout the storage. However, no changes in sulfhydryl content were obtained in actomyosin solution added with sodium dodecyl sulfate (30 mmol/g protein), urea (30 mmol/g protein), or b-mercaptoethanol (30 mmol/g protein) (p>0.05). No changes in Mg2+, or Mg2+-Ca2+-ATPase activities of actomyosin were found in the absence or presence of chemical reagents (p>0.05). Nevertheless, a significant decrease in Ca2+-ATPase activity and a significant increase in Mg2+-EGTA-ATPase activity were observed (p<0.05) with a loss of Ca2+-sensitivity when the storage time increased. For seabass muscle kept in ice, slight physicochemical changes in actomyosin were observed, though the muscle underwent degradation
as well as decomposition. However, Ca2+-ATPase activity decreased, whereas the Mg2+-EGTA-ATPase activity increased significantly after 10 days of storage (p<0.05). The results suggest that free acto-myosin undergoes denaturation to higher extent, compared to the intact actomyosin.
During frozen storage, the significant decrease in Ca2+-ATPase activity and sulfhydryl content in free and intact actomyosin was obtained, especially after 4.5 months of storage at -18oC (p<0.05). However, less changes were found in the samples stored at -80oC. Lysosomal a-glucosidase (AG)
and b-N-acetylglucosaminidase (NAG) activities increased significantly (p<0.05) to higher extent with the slow freezing and thawing rate. This suggests the leakage of lysosome caused by improper freezing and thawing. Moreover, muscle proteins were prone to physicochemical changes as well as cell damages as freeze-thaw cycles increased. Therefore, rapid freeze-thawing without repetition is
a means to preserve seabass muscle proteins.
Proteinase in seabass muscle was characterized to be cysteine proteinase based on inhibitor study. Optimum caseinolytic activity was observed at pH 6.0 and 50oC. Reducing agent effectively increased the activity, while sulfhydryl blocking agent showed the potent inhibition of activity. Sodium chloride decreased the proteinase activity in a concentration dependent manner. The endoge-nous proteinase resulted in the autolysis of seabass muscle incubated at 50 and 60oC.
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