Protein metabolism in lung: use of isolated perfused lung to study protein degradation

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Abstract
Isolated, perfused rat lung was used to study protein degradation. Proteins were labeled in vivo for 10 min or for 5 h using L-[U-14C]phenylalanine. When prelabeled lungs were perfused in vitro, virtually all of the acid-soluble and acid-insoluble radioactivity in the tissue and perfusate remained as phenylalanine. Protein degradation was measured as the accumulation of free [14C]phenylalanine in the perfusate; during the time this accumulated the amount of intracellular free phenylalanine and the free phenylalanine space remained constant. Proteins labeled during 10 min had a constant rate of degradation between 45 and 90 min of perfusion (about 11%/h); those labeled during 5 h had a constant rate of degradation for 90 min (about 3%/h). The percent dry lung weight did not change during the perfusion. Measurable proteolysis rates of rapid and slowly turning over proteins can be obtained while the lung is virtually free of edema. This system should allow studies on the modulation of proteolysis in intact lung under defined conditions.