(1) Carp ordinary muscle contained TGase activity of 1 .5unit per g of wet weight which was easily extractable with a solution at low ionic strength . The TGase, partially purified by DEAE-cellulose and gel filtration chromatographies , showed molecular weight of about 80,000 and Ca2+-requirement for full activation. These properties characterize the enzyme as a "tissue" TGase .(2) It was observed that rate and extent of TGase-catalyzed incorporation of MDC into myosin B and myofibrils were lower than those into acetylated casein .(3) The reactivity of TGase on carp myosin B and myofibrils was fluctuated by the conformational alteration of the substrate proteins; the extent of MDC incorporation appeared to be 2.4-fold elevated in "soluble myosin B" than "insoluble myosin B" or myofibrils at identical amount of the enzyme.(4) MDC was preferentially labeled on myosin heavy chain, actin, and troponin-T among the constituent proteins of myofibrils.

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Thermal gelation of salted squid mantle muscle paste was studied in relation to endogenous proteases and transglutaminase. Myosin in the paste was preferentially degraded into 130-kDa and 90-kDa fragments at an optimum temperature of 30°C. Degradation was inhibited with EDTA or 1,10-phenanthroline, suggesting the presence of metalloproteases. Myosin degradation was markedly reduced above 40°C. Although 10 mM Ca 2+ increased cross-linking of myosin heavy chains by activating the endogenous transglutaminase, setting effect on thermal gelation of the paste was offset by degradation induced by simultaneously activated calpains. Ca 2+ and the calpain inhibitor, E64, significantly improved the breaking strength and strain of thermal gels preincubated at 40°C.

The cross-linking reaction was performed in the medium containing fish myofibrillar proteins, 0.5 M (or 0.1 M) NaCl, and 5 mm CaCl2 at pH 7.5 and 25•Ž with transglutaminases from carp muscle and the culture filtrate of a strain of Streptoverticillium mobaraense. Carp enzyme (CTGase) preferentially polymerized myosin heavy chains among individual myofibrillar components. Microbial enzyme (MTGase) also rapidly cross-linked myosin heavy chains at different sites from those with CTGase. Con nectin (titin) was polymerized much faster than myosin heavy chain in soluble and aggregated states. The aggregated connectin was also cross-linked with CTGase at slow rate. Both enzymes could not cross-link actin molecules at all, though an amine (MDC) was incorporated to actin, suggesting the presence of active glutaminyl residue(s) as acyl donor. Using fluorescent probes, IAEDANS-labeled actin and myosin, it was revealed that they were cross-linked with connectin by the action of MTGase and consequently their heteropolymers with huge molecular mass were produced together with connectin homopolymers.