Clearance of chicken cystatin from the rat circulation

Chicken cystatin (mixed form) was prepared from egg white. Radioactively labeled preparations were administered intravenously to rats. 125I-cystatin disappeared from the rat circulation with a half-life of approximately 73 min. The radiolabeled inhibitor was rapidly taken up by the kidneys. Percoll density gradient analysis showed that it was incorporated into lysosomes. Within 24 hr after the injection of 125I-cystatin, 25% of the administered radioactivity was recovered in the urine, but only 2% was in the protein-bound form.