The mechanisms and regulatory factors involved in cardiac proteolysis are incompletely understood. Agents that interfere with lysosomal function (e.g., chloroquine, leupeptin, methyladenine) cause a 25-30% reduction in the overall rate of protein degradation. In the same hearts, however, the rate of myosin breakdown remains unchanged. Disaggregation of micro-tubules with colchicine is accompanied by a 15% reduction in the rate of degradation of total protein and of myosin. In the same hearts, the degradation of "organellar" protein, including mitochondrial cytochromes, is reduced by over 30%. Thus, it appears that the degradation of different classes of cardiac proteins may be accomplished and regulated by different processes. Lysosomes are important in overall proteolysis, but appear not to be involved in the regulation of myosin breakdown. Microtubules are also involved in the proteolytic process, and appear to be especially important for the breakdown of proteins from mitochondria and perhaps other organelles.
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