Sarcomere length influences proteolysis
Abstract
The overall objective of this research was to determine the role of sarcomere length on postmortem proteolysis as it relates to beef tenderness. The first two studies established that sarcomere length influences postmortem proteolysis of troponin T (TnT) and titin in excised bovine semitendinosus muscles. Muscles were removed from carcasses and allowed to proceed through rigor under different levels of stretch. Samples with longer sarcomeres had lower Warner-Bratzler shear force values than shortened samples. Muscle homogenates were subjected to SDS-PAGE and Western blotting procedures. Degradation of TnT and titin occurred earlier during postmortem storage in samples with long sarcomeres. However samples from both treatments exhibited the same extent of degradation following 10 d of storage. These results indicate structural changes induced by increased thick and thin filament overlap limit substrate access of proteases. The objective of the third study was to determine the role of sarcomere length on the in vitro proteolytic degradation of TnT in isolated myofibrils. TnT was degraded to a greater extent in myofibrils with long sarcomeres incubated with exogenous μ-calpain, supporting data from excised muscle samples. Additionally, immunoblots of purified troponin digested with μ-calpain revealed similar degradation products to digested myofibrils indicating that proteolytically susceptible degradation sites on TnT were not blocked by the formation of actomyosin crossbridges. Using the same digestion procedure, the objective of the fourth study was to determine the influence of muscle type on proteolysis at a constant sarcomere length. Analysis of western blots of myofibrils isolated from bovine masseter, semitendinosus and cutaneous trunci muscles revealed proteolysis occurred to a lesser extent in samples from the masseter indicating TnT isoforms differ in their susceptibility to calpain protease. Taken together, these data indicate that postmortem proteolysis is influenced by thick and thin filament overlap whereby degradation of proteins is limited in short sarcomeres. Most importantly, these results highlight the importance of accounting for sarcomere length in future studies focused on proteolysis and meat tenderness.
Degree
Ph.D.
Advisors
Gerrard, Purdue University.
Subject Area
Livestock
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