Effect of paratropomyosin on the increase in sarcomere length of rigor-shortened skeletal muscles

Paratropomyosin is a myofibrillar protein believed to weaken rigor linkages formed between actin and myosin. Using glycerinated fibers of rabbit psoas muscles, we studied the effect of paratropomyosin on the weakening of rigor linkages, which was followed in terms of the increase in sarcomere length of rigor-shortened muscles. The rigor tension developed was reduced to about 65% of the initial value within 10 min after the addition of purified paratropomyosin, whereas it remained constant for at least 3.5 h in control fibers. Paratropomyosin showed a stronger effect on the increase in sarcomere length of passively stretched fibers, which developed weaker rigor-tensions. The purpose of our research was to establish a rigor solution which would best permit the observation of the workings of paratropomyosin. The most successful rigor solution contained 0.2–0.25 M KCl, pH 5.5, at 5–10°C. Under these conditions, the sarcomere length was easily increased from 2.4 to 3.6 μM, if rigor-contracted fibers were passively stretched after the addition of purified paratropomyosin. Because the experimental conditions coincide well with those of postmortem muscles, it is very probable that paratropomyosin plays an important role in restoration of the sarcomere length of rigor-shortened muscles, resulting in tenderization of meat during postrigor ageing.