The kallikrein-kinin and renin-angiotensin systems in the kidney of an African lungfish, Protopterus annectens

Renin-like activity (RLA), angiotensin I converting enzyme-like (ACELA), and kallikrein-like activity (KLA), activities of the key enzymes of renin- angiotensin and kallikrein-kinin systems, were sought in the kidney of the African lungfish Protopterus annectens during the aquatic phase. RLA, examined by RIA (using porcine angiotensinogen as substrate), was 0.38 ± 0.05 ng angiotensin I/mg protein/hr. ACELA and KLA were investigated in assays spectrophotometrically. ACELA, measured at 37 and at 20°, was, respectively, 1.55 ± 0.55 and 0.61 ± 0.23 nmol hippurate/min/mg protein. KLA was 7.34 ± 0.93 mU/mg protein in the crude kidney extract and 31.05 ± 7.50 mU/mg protein after electrophoretic purification. Renal kininogenase activity was inhibited by 100% by D-Phe-Phe-Arg-chloromethyl ketone (10 μM), 98% by phenylmethylsulfonyl fluoride (2 nM), and 91% by aprotinin (1000 kIU). The apparent molecular weight of the renal kininogenase on SDS-PAGE was 27,000 Da. Both the renal enzyme and the purified glandular kallikrein, used as a control, have the same mobility on polyacrylamide gel electrophoresis. Immunoreactivities toward angiotensin II and bradykinin were localized by double immunostaining in the same cells of the proximal tubules. Putative angiotensin II receptors were demonstrated immunohistochemically, in the supranuclear region of proximal tubular cells, using an antibody to the sequence between amino acids 225 and 237 of the mammalian AT₁ receptor.