Characterization and origin of immunoreactive glandular kallikrein in rat plasma

Abstract

A radioimmunoassay (RIA) for rat urinary kallikrein (RUK), capable of detecting 200 pg of antigen, has been developed and used to help characterize the nature and origin of immunoreactive glandular kallikrein in rat plasma. Although rat plasma did not cross-react in a parallel manner in the RIA, a highly significant (P < 0.001) 4-fold increase in antigen was seen after bilateral nephrectomy. To further investigate this finding, plasmas were gel filtered and three peaks of antigen were detected in both sham-operated and nephrectomized rats. The first two peaks appear to represent inhibitor complexes, and the antigenic contents of these peaks were similar in both nephrectomized and sham-operated animals. In contrast, the third peak showed immunological identity to RUK, and the antigenic content of this peak was increased about 6-fold in nephrectomized rats compared to sham-operated controls, hence supporting the data obtained with whole plasmas. In pancreatectomized rats, the antigenic content of whole plasma and the profile obtained after gel filtration did not differ from those of sham-operated controls. Bilateral submandibular/sublingual gland excision, however, caused significant reduction in whole plasma antigen levels and in the antigenic content of Peak 3 when compared to controls. Peak 3 was indistinguishable from purified RUK and rat submandibular kallikrein by gel filtration and behaved like a typical glandular kallikrein on ion-exchange chromatography. Purified Peak 3, however, had no activity against kininogen or the chromogenic substrate S-2266 and showed a consistently smaller molecular weight ( M r, 29,500) on sodium dodecylsulfate-polyacrylamide gel electrophoresis than that of RUK (32,000). Hence, glandular kallikrein circulates in rat plasma in an inactive form. The submandibular and sublingual glands appear to be a major source of the plasma antigen, and the kidney plays an important role in the clearance and/or metabolism of glandular kallikrein from plasma.