Biochemical studies on precatorine. (III) Effect of tryptophan and its N-methyl derivatives on liver glycogen.
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Carbohydrate depletion during exercise was measured in the liver, in the three different types of skeletal muscle, and in the blood of exercise-trained and untrained rats. The acute exercise test consisted of 45 min of treadmill running of progressively increasing intensity. The training program consisted of 6 hrs of swimming per day, 5 days per week for 14 weeks; the training induced an increase of approximately 35 percent in the respiratory capacity of gastrocnemius muscle, and a 14 percent incrase in heart weight. Glycogen stores in fast-twitch red, fast-twitch white, and slow-twitch red types of skeletal muscle, were depleted significantly more slowly in the trained than in the untrained animals during the treadmill exercise test. Resting glycogen stores in the liver were higher and were depleted more slowly during exercise in the trained than the untrained animals. Blood lactate concentration was significantly lower in the trained than in the untrained rats at the end of the exercise test. These results provide evidence that endurance exercise training induces adaptation which protect against the depletion of glycogen from the liver and from the tree types of skeletal muscle during prolonged exercise.
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1. The two forms of glycogen phosphorylase were purified from human liver, and some kinetic properties were examined in the direction of glycogen synthesis. The b form has a limited catalytic capacity, resembling that of the rabbit liver enzyme. It is characterized by a low affinity for glucose 1-phosphate, which is unaffected by AMP, and a low V, which becomes equal to that of the a form in the presence of the nucleotide. Lyotropic anions stimulate phosphorylase b and inhibit phosphorylase a by modifying the affinity for glucose 1-phosphate. Both enzyme forms are easily saturated with glycogen. 2. These kinetic properties have allowed us to design a simple assay method for total (a + b) phosphorylase in human liver. It requires only 0.5 mg of tissue, and its average efficiency is 90% when the enzyme is predominantly in the b form. 3. The assay of total phosphorylase allows the unequivocal diagnosis of hepatic glycogen-storage disease caused by phosphorylase deficiency. One patient with a complete deficiency is reported. 4. The assay of human liver phosphorylase a is based on the preferential inhibition of the b form by caffeine. The a form displays the same activity when measured by either of the two assays.
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