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Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755-3844
Fully grown male CD-1 mice, fed a protein-free
diet for 3 days, received 1 g of starch with or without 300 mg casein
by intragastric intubation. We surveyed the acute effects of these
nutrients on protein synthesis in all tissues (by extrapolating to
infinity the incorporation of radioactive leucine after its injection
in massive doses) and protein degradation in skeletal muscle and liver
(by the accumulation of bestatin-induced peptide intermediates). Muscle
proteolysis was the major source of N during depletion. Compared with
postabsorptive animals, starch suppressed muscle protein loss
(synthesis +21%, degradation
24%,
P < 0.01) and stimulated hepatic
proteolysis (degradation +28%, P < 0.01). Addition of casein to the starch was anabolic in liver
(synthesis +41%, degradation
33%,
P < 0.01), gastrointestinal tract,
pancreas, and skin (synthesis +38, +69 and +38%, respectively,
P < 0.01) but had no effect on
muscle. Protein turnover proved uniquely sensitive to the dietary
supply of carbohydrates in muscle and to the endogenous or exogenous
supply of amino acids in liver.
protein turnover; protein synthesis; protein degradation
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