Distribution and elimination of intravenously injected urinary trypsin inhibitor

Elimination of human urinary trypsin inhibitor (UTI) after intravenous injection of 125I-labelled UTI was followed by serial plasma and urine samples in three male volunteers. The plasma half-life of 125IUTI during 0-3 h after injection was 33 min and during the following 4 hours the half-life was 2 hours. Free, biologically active inhibitor was found in the urine during the first four hours after injection. Most of the radioactivity in the urine, however, corresponded to free 125I probably released during the degradation of UTI in the kidney. The distribution of UTI was studied after injection of 125IUTI in rats by measurement of radioactivity in excised organs. Fifteen min after the injection 44% of the radioactivity was found in the kidneys and 9% in the liver, implying that the kidneys are the primary site of UTI metabolism.