Metabolism of Branched-Chain Amino Acids by Rat Skeletal Muscle: Regulation of Branched-Chain Alpha-Keto Acid Dehydrogenase

Catabolism of the essential branched-chain amino acids (BCAA) by skeletal muscle of the rat was investigated. In diaphragm muscles incubated in vitro oxidation and transamination of leucine or valine were accelerated by diabetes or starvation overnight. In the presence of pyruvate, transamination of BCAA by diaphragms was increased; oxidation of BCAA by diaphragms from diabetic or starved rats was increased by pyruvate but was inhibited in diaphragms of normal, fed rats. The effects on transamination were verified by direct measurements of the alpha-ketoisocaproate (KIC) product of leucine transamination. The effect of pyruvate on KIC oxidation by diaphragms was similar to the effect on leucine oxidation. Branched-chain alpha-keto acid dehydrogenase (BCDH) catalyzes the committed step in BCAA catabolism, its activity is regulated by a reversible phosphorylation-dephosporylation cycle. A method to measure active (initial) and total (active + inactive) BCDH in a soluble preparation from muscle was developed. Initial BCDH activity in fed, postabsorptive rats was about 1 nmol/min/g muscle. Total activity was ~35 nmol/min/ g muscle; thus ~2% of BCDH was in the active state. Intravenous infusion of leucine rapidly increased BCDH activity in a dose-dependent fashion; activity subsequently declined in parallel with plasma leucine concentration. Valine had no effect on BCDH activity while isoleucine was effective only at high doses. BCDH activity increased after 25% or 50% but not 9% protein meals in rats fed an adequate protein diet. Feeding a 50% protein diet chronically increased postabsorptive BCDH. Dose-dependent response of BCDH to leucine was blunted in rats fed 9% protein chronically. In insulinopenic diabetic rats plasma BCAA and muscle BCDH activity were increased. Insulin therapy or adrenalectomy of diabetic rats decreased plasma BCAA but BCDH activity remained elevated. Starvation increased BCDH activity significantly only after four days. BCDH activation in response to increased leucine was blunted in starved or diabetic rats. Conclusions: In normal fed rats, muscle BCDH activity increases in response to hyperleucinemia from meal consumption or infusion. The response to hyperleucinemia is attenuated by diabetes, starvation or consumption of a low protein diet. In diabetes increased muscle BCAA oxidation results from increased BCAA transamination and increased BCDH activity

Modulation of Rat Skeletal Muscle Branched-Chain α-Keto Acid Dehydrogenase In Vivo Effects of Dietary Protein and Meal Consumption

The effects of dietary protein on the activity of skeletal muscle branched-chain a-keto acid dehydrogenase (BCKAD) were investigated. BCKAD is rate-limiting for branched-chain amino acid (BCAA) catabolism by muscle; its activity is modulated by phosphorylation-dephosphorylation. In rats fed an adequate protein (25% casein) diet, BCKAD was - 2% active postabsorptively and increased to 10% or 16% active after a 25% or 50% protein meal, respectively. Prolonged feeding of a 50% protein diet increased postabsorptive BCKAD activity to 7% with further increases to 40% active postprandially. On a low protein (9% casein) diet BCKAD remained - 2% active regardless of meal-feeding. Dose-dependent activation of BCKAD by intravenous leucine in postabsorptive rats was blunted by a low protein diet. We conclude that excesses of dietary protein enhance the capacity of skeletal muscle to oxidize BCAA, muscle conserves BCAA when protein intake is inadequate, and skeletal muscle may play an important role in whole-body BCAA homeostasis

The Effect of High Glucocorticoid Administration and Food Restriction on Rodent Skeletal Muscle Mitochondrial Function and Protein Metabolism

Glucocorticoids levels are high in catabolic conditions but it is unclear how much of the catabolic effects are due to negative energy balance versus glucocorticoids and whether there are distinct effects on metabolism and functions of specific muscle proteins.We determined whether 14 days of high dose methylprednisolone (MPred, 4 mg/kg/d) Vs food restriction (FR, food intake matched to MPred) in rats had different effects on muscle mitochondrial function and protein fractional synthesis rates (FSR). Lower weight loss (15%) occurred in FR than in MPred (30%) rats, while a 15% increase occurred saline-treated Controls. The per cent muscle loss was significantly greater for MPred than FR. Mitochondrial protein FSR in MPred rats was lower in soleus (51 and 43%, respectively) and plantaris (25 and 55%) than in FR, while similar decline in protein FSR of the mixed, sarcoplasmic, and myosin heavy chain occurred. Mitochondrial enzymatic activity and ATP production were unchanged in soleus while in plantaris cytochrome c oxidase activity was lower in FR than Control, and ATP production rate with pyruvate + malate in MPred plantaris was 28% lower in MPred. Branched-chain amino acid catabolic enzyme activities were higher in both FR and MPred rats indicating enhanced amino acid oxidation capacity.MPred and FR had little impact on mitochondrial function but reduction in muscle protein synthesis occurred in MPred that could be explained on the basis of reduced food intake. A greater decline in proteolysis may explain lesser muscle loss in FR than in MPred rats