Protein degradation in trained athletes during prolonged intense exercise
Smith, Carolyn S.
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The effect of a single bout of prolonged intense exercise on plasma concentrations of 3-methylhistidine (3-MH), branched chain amino acids (BCAA), and the intermediates of BCAA metabolism, branched chain keto acids (BCKA), was investigated in endurance-trained men during a 3-hour treadmill run at 75% maximal working capacity in order to establish whether a preferential degradation of noncontractile vs. contractile proteins was occurring. Plasma concentrations of leucine (Leu) (pre = .104 ± .014 umol/mL; post = .096 ± .014 umol/mL); isoleucine (lie) (pre = .055 ± .013 umol/mL; post = .046 ± .011 umol/ml); valine (Val) (pre = .197 ± .028 umol/mL; post = .189 ± .021 umol/mL) and 3-MH (pre = trace; post = trace) were not significantly altered from resting values (jd > .04). Plasma concentrations of the BCKA were significantly elevated following exercise (£ < .04) (alpha-ketoisovaleric acid [KIV] [pre = .018 ± .009 umol/mL; post = .036 ± .009 umol/ml]; alpha- keto-beta-methylvaleric acid [KMV] [pre = .017 ± .008 umol/ml; post = .038 ± .013 umol/mL]; alpha-ketoisocaproic acid [KIC] [pre = .017 ± .002 umol/mL; post = .042 ± .013 umol/mL]). The ratio of lle/KMV and Leu/KIC were significantly decreased following the exercise bout (p < .04) (lle/KMV [pre = 4.28 ± 3.14; post = 1.38 ± .791); Leu/KIC (pre = 6.33 ± 1.14; post = 2.52 ± 1.07]). A significant increase in blood lactate levels was also observed (pre = 1.25 ± .600 mmol/L; post = 2.22 ± 1.03 mmol/L; £ < .04). These data suggest that during prolonged intense exercise (a) the body is able to maintain homeostasis with plasma amino acid levels, (b) there is a preferential degradation of noncontractile proteins, (c) BCAA metabolism is enhanced, (d) BCAA are only partially metabolized within muscle, and (e) some of the BCAA keto analogs are released into the circulation. These findings support the contemporary view which suggests that proteins and their amino acid derivatives are used along with other substrates to support the metabolic apparatus necessary for energy production during prolonged muscular exercise.