Structural aspects of rat angiotensinogen.

Rat angiotensinogen (Ao) consists of two molecular weight forms (Ao-1 and Ao-2). The amino acid composition of both forms is very similar. In addition, the immunological properties of both molecules obtained by immunodiffusion and direct radioimmunoassay are very similar as well. Beside the difference in molecular weight there is a heterogeneity in isoelectric points within and between both forms. The difference in the carbohydrate moiety between both forms is the basis of the suggestion that the main difference in weight and charge is due to the presence or absence of one oligosaccharide chain. The twin bands of Ao-2 in IEF which have the same amount of sialic acid, can be separated on ConA Sepharose CL-6B, indicating that different types of oligosaccharide chains are attached to Ao-2. The structural features of angiotensinogen have been studied by circular dichroism. The structure of angiotensinogen consists of 17% alpha-helix, 37% beta-structure and 45% irregular structure. Concentrations of dithiothreitol (DTT) of up to 2o mM cause a decrease in regular structure at neutral pH and an increase in regular structure at pH 9.5. There is no change in the amount of angiotensin I cleaved by the enzyme renin. However the binding of angiotensinogen to a specific rabbit antibody is decreased following treatment with 1o or 2o mM DTT.