The Distribution of Angiotensinogen in Dog Brain Studied by Cell Fractionation*

Cerebrospinal fluid contains angiotensinogen; however, its origin and role are not known. The possibility that brain cells contain angiotensinogen in granules was examined by cell fractionation. Subcellular fractions of midbrain and forebrain of dogs generated angiotensin I over a period of 60 min when incubated with renin at 37 C, pH 7.4. The results for midbrain and forebrain were virtually identical. The highest specific concentration of angiotensinogen was in the soluble fraction. Some angiotensinogen was present in granule fractions, but it was shown by isopycnic gradient centrifugation and in control experiments using 125I-labeled human serum albumin and an 125I-labeled preparation of dog angiotensinogen that this could be accounted for entirely by soluble contaminant and nonspecific uptake by granules, probably lysosomes, during the fractionation procedure. The concentration in whole tissue was ca. 45 pmol/g tissue and calculations based on the concentration in cerebrospinal fluid suggest that all of the angiotensinogen in brain could be extracellular. The apparent absence of specific particle-bound angiotensinogen suggests that brain cells in these regions do not produce secretory granules containing the protein. Furthermore, the results indicate that a reaction of angiotensinogen with cathepsin D, an enzyme that accounts for at least a large part of the “renin-like” activity in brain, is doubtful, as this lysosomal enzyme would be capable of hydrolyzing angiotensinogen only in the acidic environment of the lysosome.