Abstract: The objective of this study was to investigate the effects of phosphorylation on titin degradation at different Ca2+ concentrations in vitro. Titin was extracted from ovine longissimus lumborum muscles. Protein kinase A (PKA) and alkaline phosphatase (AP) were individually added to titin in vitro to change its phosphorylation level. The samples were then incubated with μ-calpain at 4 ℃ for 2 days at Ca2+ concentrations of either 0.05 or 2 mmol/L. The pH of the systems was measured and the degrees of phosphorylation and degradation of titin were determined. The results showed that there were no significant differences in pH values among the PKA, AP and control groups (P > 0.05). The relative phosphorylation levels of titin in the groups followed the decreasing order of PKA > control > AP (P < 0.05). The degradation product of about 1 200 kDa was detected in the AP group after 12 h incubation at a Ca2+ concentration of 0.05 mmol/L, but not in the other groups. The 1 200 kDa band was detected in both the PKA and control groups after 12 h, but after 0.5 h in the AP group at a Ca2+ concentration of 2 mmol/L. These observations demonstrated that the phosphorylation of titin in vitro was significantly enhanced by PKA but attenuated by AP. With the increase of calcium concentration, the degradation rate of titin became faster. The degradation of titin in the AP group was faterer than in the other groups at the same Ca2+ concentration. It was suggested that the dephosphorylation promoted titin degradation.

Key words: titin, protein kinase A, alkaline phosphatase, phosphorylation, degradation