The objective of this study was to investigate changes occurring in μ/
m-calpain in post mortem chicken muscles and to determine the origin of the unknown bands found in
calpain casein zymography. The unknown bands were reported with slightly greater mobility compared to conventional μ/
m-calpain bands in
casein zymography. Identification of these bands was accomplished using native
polyacrylamide gel electrophoresis, liquid chromatography tandem mass spectrometry and with
protein phosphatase treatment. Results showed that the unknown bands were corresponding to μ/
m-calpain, and dephosphorylation by
protein phosphatase did not change their appearance. The
calpain samples were then incubated with various concentrations of Ca2+ to determine the relationship between changes in μ/
m-calpain and the appearance of the unknown bands. The products of μ/
m-calpain partial
autolysis were found to be consistent with the appearance of the unknown bands. Therefore, the appearance of these bands did not result from phosphorylation of μ/
m-calpain as previously hypothesized, but from partial
autolysis of μ/
m-calpain. Also their presence suggests that μ/
m-calpain undergoes partial
autolysis during aging which may play certain roles in meat quality improvement.