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In fast- and slow-twitch limb and heart muscles of cachectic rats, redox balance and muscle structure were explored. The nature of the oxidatively modified proteins also was identified in these muscles. Reactive carbonyls, hydroxynonenal (HNE)- and malondialdehyde (MDA)-protein adducts, and antioxidant enzyme levels were determined in limb and heart muscles of cachectic (7 days after inoculation of Yoshida AH-130 ascites hepatoma) and control rats. Moreover, carbonylated proteins were identified (proteomics), and fiber-type composition evaluated (morphometry) in these muscles. In cachectic rats, compared with the controls: (a) HNE- and MDA-protein adducts levels were greater in gastrocnemius, tibialis anterior, soleus, and heart; (b) in the gastrocnemius, type II fiber size was reduced, and the intensity of carbonylated protein immunostaining was significantly greater in these fibers; and (c) proteins involved in glycolysis, ATP production and distribution, carbon dioxide hydration, muscle contraction, and mitochondrial metabolism were significantly more carbonylated in limb and heart muscles. Cancer cachexia alters redox balance in fast- and slow-twitch limb and heart muscles of rats, inducing increased oxidative modifications of key proteins involved in muscle structure and function. Additionally, it induces a reduction in type II fiber size in the gastrocnemius, which is associated with increased protein oxidation.