Kininase one-an'-a-half: the newest member of the kininase family.

Abstract

A kininase I-like enzyme (carboxypeptidase) was purified to homogeneity from human urine and compared to the 48,000 mol. wt. (48K) active subunit of carboxypeptidase N. The urinary carboxypeptidase had a mol. wt. of 73,000 in gel filtration and 76,000 in SDS-polyacrylamide gel electrophoresis. It had a pH optimum of 7.0 and differed from the 48K subunit in stability, susceptibility to trypsin, and enzymatic activity. The urinary enzyme did not cross-react with antibody to carboxypeptidase N in "Western blotting". Urine from a patient genetically deficient in plasma carboxypeptidase N (21% of normal) contained normal levels of urinary carboxypeptidase with similar properties to that from pooled human urine. Membrane fractions from several tissues contained a similar carboxypeptidase activity. The activity was highest in a microvillous membrane fraction from human placenta (65 nmol/min/mg with Bz-Gly-Lys as substrate). High specific activities were also found in membrane fractions of human kidney (18 nmol/min/mg) and lung (8 nmol/min/mg). The membrane-bound enzyme was distinguished from lysosomal and catheptic carboxypeptidases as well as "enkephalin convertase" by the use of specific inhibitors. These results show that urine contains a carboxypeptidase capable of cleaving arginine or lysine from the C-terminus of peptides. The enzyme does not arise from plasma carboxypeptidase N, but may be released into the urine from the renal brush border.