Comparative quantification of rat brain and pituitary angiotensin-converting enzyme with autoradiographic and enzymatic methods.

Angiotensin-converting enzyme (ACE, kininase II, EC 3.4.15.1) was quantified in selected areas of the rat brain by 3 different methods. The enzyme activity was quantified by an enzymatic assay in homogenates of discrete brain areas with the use of the artificial substrate hippuryl-His-Leu-[14C]glycine. Binding of the specific ACE inhibitor [125I]351A was quantified after incubation of thin brain sections followed by quantitative autoradiography with comparison to [125I]standards. The antigenic sites of the enzyme were quantified by immunoautoradiography, using a polyclonal antibody to ACE in combination with [125I]protein A followed by quantitative autoradiography. There was a good correlation between the ACE activity, as determined by the quantitative enzymatic method, and both the enzyme inhibitor binding and the [125I]protein A binding, as determined by quantitative autoradiography, in brain areas and in anterior and posterior lobes of the pituitary gland. The present data demonstrate that ACE activity and ACE binding can be quantified in discrete tissue areas with different methods in a variety of conditions and with a high degree of accuracy.