An assay of microsomal membrane-associated proteasomes demonstrates increased proteolytic activity in skeletal muscle of intensive care unit patients

Background & Aims: Muscle wasting during critical illness is generally believed to be an increase in muscle protein degradation mediated by the proteasome proteolytic pathway. Polyubiquinated proteins are recognised and degraded by the 26S multicatalytic proteasome complex. Animal models for various catabolic conditions have shown increased expression of mRNA:s for several enzymes and subunits in the ubiquitin–proteasome pathway as well as an increase in proteasome activity. The aim of this study was to measure the proteolytic activity of the proteasome in human skeletal muscle. We investigated the proteasome activity in leg muscle biopsies from 7 critically ill patients and from a reference group of 7 age and sex matched patients by a method that could also be suitable for repetitive measurements of intensive care unit patients in future studies. Methods: Proteasomes were isolated by ultracentrifugation and the fractions containing cytosolic soluble and membrane-bound proteasomes were, respectively, incubated with a fluorogenic peptide substrate to assess the chymotrypsin-like peptidase activity. Results: In the critically ill the proteasome activity in the membrane-bound fraction of proteasomes was 30% higher compared to the reference group (P<0.02), whereas no difference was seen regarding the soluble fraction. Conclusion: The results indicate that there is an altered distribution of proteasome activity at the subcellular level in skeletal muscle of critically ill intensive care unit patients.