Metabolism of [3H]leupeptin by rat liver

Tritium-labeled leupeptin was used to study how this tripeptide proteinase inhibitor interacts with the liver, including the mechanism of its transport into the cell, its subcellular distribution after uptake, and its metabolism once in the tissue. Experiments were done in situ and in a perfused liver. At low concentrations (1 to 10 microM) the uptake of radioactive inhibitor was competed by chemically reduced leupeptin. At high concentrations at least up to 400 microM the uptake was directly proportional to the external concentration of tripeptide. Entry into the tissue essentially stopped at low temperature (less than 21 degrees C). [3H]Leupeptin initially was located in the soluble fraction of the liver homogenate and by 15 to 30 min became concentrated in the lysosome-rich fraction. During 2 h of perfusion almost 50% of [3H]leupeptin that had entered the liver was secreted intact into the bile. In addition, a portion of the leupeptin that remained in the liver was degraded during this time period.