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Reactive oxygen species (ROS) are thought to promote muscle atrophy in chronic wasting diseases, but the underlying mechanism has not been determined. Here we show that H[sub 2]O[sub 2] stimulates ubiquitin conjugation to muscle proteins through transcriptional regulation of the enzymes (E2 and E3 proteins that conjugate ubiquitin to muscle proteins. Incubation of C[sub 2]C[sub 12] myotubes with 100 µM H[sub 2]O[sub 2] increased the rate of [sup 125]I-labeled ubiquitin conjugation to muscle proteins in whole cell extracts. This response required at least 4-h exposure to H[sub 2]O[sub 2] and persisted for at least 24 h. Preincubating myotubes with cycloheximide or actinomycin D blocked H[sub 2]O[sub 2] stimulation of ubiquitin-conjugating activity, suggesting that gene transcription is required. Northern blot analyses revealed thai H[sub 2]O[sub 2] upregulates expression of specific E3 and E2 proteins that are thought to regulate muscle catabolism, including atrogin1/MAFbx, MuRF1, and E2[sub 14k]. These results suggest that ROS stimulate protein catabolism in skeletal muscle by upregulating the ubiquitin conjugation system. [ABSTRACT FROM AUTHOR]