A direct radioimmunoassay for human urinary kallikrein

A protein-binding radioimmunoassay (RIA) has been developed for human urinary kallikrein (HUK). HUK was purified to apparent homogeneity and rabbit anti-human urinary kallikrein serum was prepared. Radio-iodination of HUK was performed in 20 microgram batches with 1 mCi of 125I-Bolton-Hunter reagent in 0.1 M sodium borate buffer, pH 8.5, at 0 degree C. The 30 min reaction was terminated by adding 0.5 ml of 0.2 M glycine. Radiolabelled HUK was purified on a Sephacryl S-200 column (1x 50 cm) in 0.05 M Tris.HCl plus 0.2% gelatin, pH 7.4 (RIA buffer). 125I-HUK emerged as a single, uniform peak at Ve/Vo of 1.3. The RIA reaction mixture contained 15,000 cpm of 125I-HUK, 0.1 ml of diluted anti-kallikrein serum, 0.1 ml unlabelled kallikrein (1-100 ng) or the unknown sample and RIA buffer to a total of 0.5 ml. The reaction mixture was incubated at 22 degrees C for 18 hr, and 100 microliter to goat-anti-rabbit IgG serum was added to each tube. The tubes were incubated 18 hr at 4 degrees C, centrifuged 4,000 rpm for 30 min and, after decantation, the precipitate was counted for 125 I. The RIA reported here is sufficiently sensitive to detect 1 ng of kallikrein and should offer a useful technique to determine alterations of the kallikrein-kinin system in response to stimuli; both physiologic and pathologic.