Elevated rate of collagen solubilization and postmortem degradation inmuscles of lambs with high growth rates: Possible relationship with activity of matrix metalloproteinases

The extracellular matrix, composed mainly of collagen, is considered responsible for the residual toughness of meat. Matrix metalloproteinases (MMP) responsible for the degradation of connective tissue are found in most tissues, but their participation in meat aging has not been tested. We recently showed that skeletal muscle has multiple MMP activities, as well as regulators and tissue inhibitors of metalloproteinases. Here we present the first observations of physiologic and postmortem variation of MMP activities in muscle. Growing lambs were offered two levels of intake: hay + concentrate for lambs with high growth rate (average daily gain > 250 g) and hay only for those with low growth rate (average daily gain < 25 g). At slaughter and at 21 d of postmortem aging of longissimus and semimembranosus muscles, we studied collagen content, collagen solubility, free hydroxyproline (OH-pro), and levels of latent and active forms of a matrix metalloproteinase (MMP-2) by gelatin zymography. Our results demonstrate the presence of an active isoform of MMP-2 in lamb muscle. Its level was higher (+90%, P < 0.01) in lambs that expressed a high growth rate. Activity of MMP-2 was also present at 21 d postmortem, at levels similar to those detected at slaughter. At slaughter and at 21 d, all muscles contained latent MMP-2 and the quantity of proenzyme was greater than that present in the activated form. The levels of free OH-pro in muscles of lambs with high growth rate increased significantly (P < 0.001) over 21 d from 3.75 to 5.08% of total collagen, and this was significantly related to the level of active MMP-2 at slaughter. By contrast, the amount of free OH-pro in muscles of lambs with low growth rate was not different at 21 d (1.63% of total OH-pro) than it had been at slaughter (1.84% of total OH-pro). These results suggest that collagen degradation all the way to free amino acids occurs postmortem in muscle and that there are active MMP simultaneously present that may account for this catabolism. The growth rate of animals at slaughter influences collagen turnover in vivo, as well as postmortem collagen degradation.