Effect of ageing on the properties of myofibrils of rabbit muscle

The effect of ageing of rabbit muscle at 4° and 15–18° on the extractability and adenosine triphosphatase activity of the myofibrils has been examined. The amount of protein extracted by M‐KCL‐4 mM sodium glycerophosphate (pH 6.2) and by 0.1 M sodium tetrapyrophosphate‐4 mM MgCl2‐10 mM‐KH2PO4 (pH 7.2) increased as the muscle aged. By using the amount of Ca2+ adenosine triphosphatase extracted, i.e. the enzyme associated with myosin, as a measure of the amount of myosin in the actomyosin extracted, it was possible to show that the buffered potassium chloride did not extract all the actomyosin from the myofibrils of pre‐rigor muscle. As the muscle aged, more actomyosin was extracted, together with some tropomyosin. Pyrophosphate, however, extracted all the myosin from the pre‐rigor muscle, and the increase in the protein extracted from aged muscle was due to actin and tropomyosin in addition to myosin. It is suggested that these changes are caused by a disruption of the Z‐band structure during ageing, perhaps due to the hydrolysis of tropomyosin by proteolytic enzymes. The specific Ca2+ adenosine triphosphatase activity of the myofibrils was unaltered by ageing but the specific Mg2+‐activated adenosine triphosphatase, i.e. the enzyme associated with actomyosin, was reduced by about one‐third. This latter result may be caused by a change in the mode of linkage of actin to myosin.