Aggregation of Myosin Heavy Chain while Grinding Surimi and Setting Its Paste

Abstract

Most recently we reported that myosin heavy chain (MHC) aggregate which could not easily be dissociated merely by simple treatment with urea was contained in the muscle proteins of set gel from Alaska pollack Theragra chalcogramma frozen surimi paste. No positive evidence, however, was given that the aggregate was formed during the setting of the paste. In this paper, we describe the MHC aggregates during the setting of the paste and also during the grinding of the surimi with NaCl. Unsalted frozen surimi of Alaska pollack (Golden Alaska Sea Foods, SA grade) was minced after thawing, ground with 3% NaCl, 1% sterilizer (Ueno Fine Chemical Co., Solmighty), and 30% water, and set at 30 degree C for 8 h in a polyvinylidene chloride casing. The surimi, paste, and set gel were dissolved in 19 volumes of 8 m urea-2% SDS-2% 2-mercaptoethanol-20 mm Tris -HCI (pH 8.0) as described previously-1) and SDS-polyacrylamide gel electrophoresis (SDS-PAGE) was carried out in the 5% gel after adjusting the protein content to 35 pg. The proteins were extracted with the above buffer from the top 5 mm portion of eight unstained disk gels after SDS -PAGE and were electrophoresed once more. Figure 1 shows the SDS-PAGE patterns for the surimi, paste, and set gel, and for the extracts from the top of the disk gels. The intensity of the MHC band was almost the same in the surimi (a) and the paste (b), but was remarkably weak in the set gel (c). Conversely, a band at the top of the disk gel, corresponding to macromolecular aggregates or polymers, was the strongest in the latter. The MHC band and that at the top reappeared along with the 66 kDa protein band in the patterns for all the extracts. The intensity of the MHC band was strong in the order of the extract from the set get (f), that from the paste (e), and that from the surimi (d), suggesting that more MHC has been contained in the aggregate in this order, or in other words that the MHC is aggregated not only during the setting of the paste but also during the grinding of the surimi with NaCl, although the aggregation is less in the latter case. Therefore, the band due to the MHC aggregate thus formed may be overlapped to that of crosslinked MHC, in which the MHCs are covalently bonded by the catalysis of transglutaminase.