Protein degradation is the process of breaking down proteins into their constituent amino acids. These amino acids contribute to the intracellular free amino acid pool, which may be exported into the plasma, directly oxidized, or reincorporated back into tissue protein (synthesis). Although most athletes think only of maximizing protein synthesis, it is equally logical to try to attenuate degradation, for net protein balance is a function of synthesis minus degradation. A bodybuilder, for example, could achieve net protein retention by decreasing degradation even without a change in synthesis.
The three main pathways for protein degradation in human skeletal muscle include the lysosomal (cathepsin) and nonlysosomal (calpain and ubiquitin) pathways. The lysosomal pathway degrades endocytosed proteins, some cytosolic proteins, hormones, and immune modulators.31 This pathway is not a major contributor to human skeletal muscle protein degradation,32 except when there is significant muscle damage and inflammation.33 The two major nonlysosomal pathways in human skeletal muscle are the (ATP)-dependent ubiquitin pathway31 and the calcium-activated neutral protease or calpain pathway.34-36 The calpain pathway is felt to play a role in skeletal muscle proteolysis during exercise.34 The ubiquitin pathway is activated after the targeting of proteins for degradation (e.g., oxidative modification). Following targeting, ubiquitin molecules are linked to lysine residues through a series of pathways catalyzed by three enzymes, termed E1, E2, and E3, and are then degraded by the 26S proteosome into peptides.31 This pathway is activated during starvation and muscle atrophy.37 Evidence suggests that activation of apoptotic pathways, particularily caspase 3, is a prerequisite for initiation of ubiquitin-proteo-some-mediated proteolysis of acto-myosin.38 It is not currently known whether endurance exercise training has an effect on the activation or content of any of the specific protein breakdown pathways.
In addition to dietary protein intake, protein degradation is the only other source of amino acid contribution to the intracellular free amino acid pool. In human skeletal muscle, at least eight amino acids (alanine, asparagine, aspartate, glutamate, iso-leucine, leucine, lysine, and valine) can be oxidized.39 During exercise, however, the branched-chain amino acids (BCAAs; isoleucine, leucine, and valine) are preferentially oxidized.39-42 The BCAAs are transaminated to their ketoacids via branched-chain aminotransferase (BCAAT), with subsequent oxidation occurring via branched-chain oxo-acid dehydrogenase enzyme (BCOAD).43,44 In the cytosol, the amino-N group is usually transaminated with a-ketoglutarate to form glutamate, which is in turn transaminated with pyruvate to form alanine45 or aminated via glutamine synthase to form glutamine. Some of the amino-N may end up as free ammonia released from muscle; however, during high-intensity contractions most of the ammonia comes from the myoadenylate deaminase pathway.46-48 The BCOAD enzyme is rate limiting in BCAA oxidation, with about 5 to 8% active (dephos-phorylated) at rest and 20 to 25% active during exercise.40,43 BCOAD activation is related to a decrease in the ATP/ADP ratio, a decrease in pH, and a depletion of muscle glycogen.48-51 The inverse correlation between BCOAD activation and muscle glycogen concentration48,49 provides theoretical support for strategies to ensure
CHO availability during exercise to attenuate BCOAD-mediated amino acid oxidation. Somewhat surprisingly, glucose supplementation during exercise does not appear to significantly attenuate BCOAD activation even though leucine oxidation was attenuated at higher protein intakes (1.8 g/kg/day).52
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