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.

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