Quantification of the calpain proteolytic system, and the effects of growth hormone enhancer and selection for glucose tolerance on the calpain proteolytic system and tenderness in lambs

Abstract

The calpain proteolytic system has been implicated in determining the tenderness of post-mortem muscle. An appreciation of factors that impact upon this system may allow commercial meat producers and processors to generate meat of greater tenderness. This thesis examines the effect of glucose tolerance and growth hormone enhancer on the activity of enzymes belonging to the cal pain system, with a view to developing a better understanding of the system.

A variety of ion exchange and hydrophobic chromatography methods used to extract calpains were compared. DEAE based methods of extraction were superior to phenyl sepharose with respect to accuracy and reproducibility of calpain separation. A protocol was developed and used in subsequent experiments.

Previously sheep had been bred selectively based upon their rates of glucose clearance. The two lines produced, displayed fast and slow clearance and also presented differing body compositions. Fast sheep generally had lower % protein but higher % fat compared to slow (p<0.05). Fast sheep also had significantly greater (p<0.05) activity of calpain I and II extracted from post-mortem skeletal muscle, and lower calpastatin. This may indicate that the lower % protein was attributable to augmented proteolytic enzyme levels. Enzyme levels were not correlated with differences in tenderness or pH of meat from these animals.

Immunogenic peptide fragments of growth hormone can associate with antibodies and generate an amplified GH response. Experiments were performed to confirm this by injecting lambs with a GH peptide. No effect on growth rate was observed, although an immune response did occur. This treatment had no effect on body composition or calpain activity, and no differences in tenderness were observed.

This work succeeded in developing a reliable protocol for the extraction and quantification of the calpain system. Heightened protease levels were correlated with lower protein content in sheep selected for fast clearance of glucose. This may indicate a function for the calpains in in vivo nutrient partitioning. The GH enhancing peptide used in this work failed to produce a change in the hormonal status of the animals, and had no effect on meat quality.