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Compartmentation of free amino acids for protein synthesis in rat liver.

ABSTRACT: The concept that a general intracellular pool serves as the sole precursor of amino acids for protein biosynthesis has been vigorously debated in recent years. To help resolve this controversy, we followed the distribution of intraperitoneally administered [(3)H]valine in the tRNA and the extracellular and intracellular compartments of rat liver. The specific radioactivity of the valine released from isolated tRNA was 2-3 times higher than that of intracellular valine, suggesting that the intracellular pool cannot be the sole precursor of amino acids used for charging tRNA. In addition, the specific radioactivity of the tRNA was only half that of the extracellular valine. Therefore it is unlikely that the valyl-tRNA is charged exclusively with amino acids derived from the extracellular pool. A model is proposed which stipulates that both extracellular and intracellular amino acids contribute to a restricted compartment that funnels amino acids towards protein biosynthesis.

SUBMITTER: Airhart J

PROVIDER: S-EPMC1168033 | biostudies-other | 1974 Jun

REPOSITORIES: biostudies-other

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Project description:BACKGROUND:A previous clinical study reported that the addition of an amylopectin/chromium complex (ACr; Velositol®) to 6?g of whey protein (WP) significantly enhanced muscle protein synthesis (MPS). Branched-chain amino acids (BCAAs) are also well-known to enhance MPS. The aim of this study was to determine if the addition of ACr to BCAAs can enhance MPS and activate expression of the mammalian target of the rapamycin (mTOR) pathway compared to BCAAs and exercise alone in exercise-trained rats. METHODS:Twenty-four male Wistar rats were randomly divided into three groups (n?=?8 per group): (I) Exercise control, (II) Exercise plus BCAAs (0.465?g/kg BW, a 6?g human equivalent dose (HED)), and (III) Exercise plus BCAAs (0.465?g/kg BW) and ACr (0.155?g/kg BW, a 2?g HED). All animals were trained with treadmill exercise for 10?days. On the day of the single-dose experiment, rats were exercised at 26?m/min for 2?h and then fed, via oral gavage, study product. One hour after the consumption of study product, rats were injected with a bolus dose (250?mg/kg BW, 25?g/L) of phenylalanine labeled with deuterium to measure the fractional rate of protein synthesis (FSR). Ten minutes later, muscle tissue samples were taken to determine MPS measured by FSR and the phosphorylation of proteins involved in the mTOR pathway including mTOR, S6K1, and 4E-BP1. RESULTS:ACr combined with BCAAs increased MPS by 71% compared to the exercise control group, while BCAAs alone increased MPS by 57% over control (p?<?0.05). ACr plus BCAAs significantly enhanced phosphorylation of mTOR, S6K1 and 4E-BP1 compared to exercise control rats (p?<?0.05). The addition of ACr to BCAAs enhanced insulin levels, mTOR and S6K1 phosphorylation compared to BCAAs alone (p?<?0.05). Serum insulin concentration was positively correlated with the levels of mTOR, (r?=?0.923), S6K1 (r?=?0.814) and 4E-BP1 (r?=?0.953). CONCLUSIONS:In conclusion, the results of this study provide evidence that the addition of ACr to BCAAs significantly enhances exercise-induced MPS, and the phosphorylation of mTOR signaling proteins, compared to BCAAs and exercise alone.

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Compartmentation of free amino acids for protein synthesis in rat liver.

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