Evidence for renin in rat brain: differentiation from other reninlike enzymes

We observed that unfractionated rat brain extract incubated with substrate at pH 6.0 yielded 12 times the quantity of angiotensin I as incubations at pH 7.4, but the enzyme activity measured at pH 6 was not primarily due to renin. To examine the existence of renin in brain, we used three methods of affinity chromatography (pepstatin-, renin-specific antibody-, and alpha-casein-Sepharose) to fractionate the angiotensin I-generating enzymes in the brain. 1) Brain extract applied to renin-specific column eluted a peak of angiotensin-releasing activity (ARA) that had a pH optimum of 6.0. This ARA was inhibited by antirenin antibody. Another peak of ARA with a pH optimum of 4 appeared in the nonbound fraction. This peak was not affected by antirenin antibody and had acid protease activity. 2) Pepstatin affinity column elution with lithium bromide yielded an early ARA peak (pH optimum 6.5), inhibited by antirenin antibody and a later peak (pH optimum 4.0) not inhibited by antirenin antibody. The latter contained acid protease activity. 3) alpha-Casein-Sepharose column also separated neutral proteases and immunoreactive renin from acid protease capable of generating angiotensin. In summary, rat brain contains a host of angiotensin I-generating enzymes that can be detected and separated as neutral and acid proteases and immunoreactive renin depending on the pH of the assay and conditions of purification. These findings indicate the presence of an enzyme with immunoidentity to renin in rat brain but do not imply local biosynthesis.