Characterization of different high molecular weight angiotensinogen forms

BACKGROUND: Angiotensinogen is the substrate for renin in the system that releases angiotensin II. This renin-angiotensin system is an important regulator of blood pressure (BP), and defects in the system are linked to the development of hypertension. Native angiotensinogen is a 62,000-dalton monomer, but various high molecular weight forms also exist, which have not been well characterized. High molecular weight angiotensinogen has been reported to be 5% of the total angiotensinogen, and increases to 60% of the total during pregnancy and hypertension. The purpose of this investigation was to study high molecular weight angiotensinogen in normal plasma. METHODS: Normal human plasma was run on a gel filtration column, and high molecular weight angiotensinogen detected by Western blotting. Further purification was by ion-exchange chromatography. In vitro polymerization of angiotensinogen was analyzed by sodium dodecyl sulfate (SDS) and native gels. RESULTS: Two forms of high molecular weight angiotensinogen were found with molecular weights of 500,000 and 250,000 daltons on gel filtration, and 140,000 and 110,000 daltons, respectively, on nonreduced SDS-polyacrylamide gel electrophoresis, and at 62,000 daltons on reduced gels. Our estimation of the amount of high molecular weight angiotensinogen present is close to that reported previously. We also describe some of the in vitro polymerization characteristics of angiotensinogen, which can be explained by angiotensinogen being a member of the serpin family of proteins. CONCLUSIONS: The angiotensinogen polymers produced in vitro might provide a model system for some of the high molecular weight forms produced in vivo, and help in understanding their function.