Tests to localize free proteolytic enzymes in vivo by 14C-cysteine-aprotinin

Aprotinin, a polyvalent protease inhibitor from bovine organs, has been labelled with 14C-cysteine in 6 M urea to produce a radioactive conjugate, without effect on the inhibitor activity. 138x10(3) dpm of radioactive product, containing 5 mg of protein (20.000 kal. inhibitor units) in 5 ml of Locke's solution were perfused in anesthetized rabbits via ascending aorta. Then the anesthetized rabbit was killed and specimens of some organs were admitted to autoradiographic analysis. Small intestine, ischiatic nerve and testis have been the most rich radiolabelled organs (+++). In kidney, stomach, large intestine, liver and eye the radiolabelled aprotinin was found in fair amounts (++); while in pancreas, spleen, spinal cord, brain, cava vein and aorta of rabbit the 14C-cysteine-Aprotinin was practically undetectable by autoradiography.