Clearance and metabolism of neurophysins by rat kidney

The half-life in the circulation of 125I-labelled porcine neurophysins, injected intravenously into rats, was determined. The radioactivity in blood collected at intervals after injection was characterized as neurophysin by polyacrylamide gel electrophoresis. The half-life of neurophysin I was 1-5 plus or minus 0-1 (S.E.M.) min and that of neurophusin II was 1-7 plus or minus 0-1 min. The metabolic clearance rate of neurophysin I and neurophysin II was 1-9 plus or minus 0-1 and 1-3 plus or minus 0-2 ml/min/200 g respectively. Clearance from the circulation was due to both equilibration with the extravescular space and rapid specific accumulation by the kidney. The neurophysins were filtered at the glomerulus. Most of the neurophysin takken up by the kidney was incorporated into lysosomes, probably in the proximal tubule and degraded to low-molecular-weight metabolites which appeared in urine and which represented the principle mode of excretion of neurophysins.