Evidence that lysosomes are not involved in the degradation of myofibrillar proteins in rat skeletal muscle

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Abstract
To examine the role of lysosomes in the degradation of skeletal-muscle myofibrillar proteins, we measured the release of N.tau.-methylhistidine from perfused muscle to starved and fed rats in the presence or absence of agents that inhibit lysosomal proteinase activity. After 1 day of starvation, the release of N.tau.-methylhistidine by perfused muscle of 4-, 8- and 24-week-old rats increased by 322, 159 and 134% respectively. On the other hand, total protein breakdown, assessed by tyrosine release, increased by 62, 20 and 20% respectively. Inhibitors of lysosomal proteinases as well as high concentrations of insulin or amino acids failed to diminish the release of N.tau.-methylhistidine by perfused muscle of starved and fed rats, despite a 25-35% inhibition of total protein breakdown. The data strongly suggest that the complete breakdown of myofibrillar proteins occurs via a non-lysosomal pathway. They also suggest that total proteolysis, which primary reflects non-myofibrillar protein breakdown, occurs at least in part within lysosomes.