Measurement of the rat urinary plasminogen activator (esterase A) by direct radioimmunoassay in urine and tissue.

Rat urinary esterase A, a plasminogen activator with kininogenase activity, was recently purified and characterized (J. Chao (1983) J. Biol. Chem. 258, 4434-4439). A sensitive radioimmunoassay for esterase A has been developed. This assay uses a rabbit antiserum in a final dilution of 1:160 000 and the purified enzyme was labelled with 125I using a lactoperoxidase method. It detects 80 pg of immunoreactive material per tube. This antiserum has some cross-reactivity with rat urinary kallikrein (approximately 5%) but a previously characterized tissue kallikrein antiserum has negligible cross-reactivity with the urinary esterase A in the assays. Therefore, kallikrein levels are measured simultaneously in all samples to obtain accurate levels of immunoreactive esterase A. Dilutions of urine or tissue homogenates showed complete parallelism with esterase A standard curves. No cross-reactivity with dog, human or monkey urine was seen. The recovery of esterase A from rat urine was 99.7 +/- 3.5%. Intra- and between-assay errors were 6.5 and 11.2%, respectively. Immunoreactive esterase A was measured and compared with kallikrein levels in rat urine, kidney, pancreas, submandibular gland, descending colon and ileum. The urinary esterase A excretion rate was reduced significantly in rats on a high sodium, compared with a low sodium diet, but not significantly increased above control by the latter. Nonetheless, a significant correlation between urinary kallikrein and esterase A excretion rate was present. This radioimmunoassay can now be used to measure esterase A levels in urine and tissue as questions have arisen about its regulation and functional significance.