Aminopeptidase A, which generates one of the main effector peptides of thebrain renin-angiotensin system, angiotensin III, has a key role in centralcontrol of arterial blood pressure

Overactivity of the brain renin-angiotensin system (RAS) has been implicate d in the development and maintenance of hypertension in several experimenta l animal models. We have recently reported that, in the murine brain RAS, a ngiotensin II (AngII) is converted by aminopeptidase A (APA) into angiotens in III (AngIII),which is itself degraded by aminopeptidase N (APN), both pe ptides being equipotent to increase vasopressin release and arterial brood pressure when injected by the intracerebroventricular (i.c.v.) route. Becau se AngII is converted in vivo into AngIII, the exact nature of the active p eptide is not precisely known. To delineate their respective roles in the c entral control of cardiovascular functions, specific and selective APA and APN inhibitors are needed to block the metabolic pathways of AngII and AngI II respectively. In the absence of such compounds for APA, we first explore d the organization of the APA active site by site-directed mutagenesis. Thi s led us to propose a molecular mechanism of action for APA similar to that proposed for the bacterial enzyme thermolysin deduced from X-ray diffracti on studies. Secondly, we developed a specific and selective APA inhibitor, compound EC33 [(S)-3-amino-4-mercaptobutylsulphonic acid], as well as a pot ent and selective APN inhibitor, PC18 (2-amino-4-methylsulphonylbutane thio l). With these new tools we examined the respective roles of AngII and AngI II in the central control of arterial blood pressure. A central blockade of APA with the APA inhibitor EC33 suppressed the presser effect of exogenous AngII, suggesting that brain AngII must be converted into AngIII to increa se arterial blood pressure. Furthermore, EC33, injected alone i.c.v. but no t intravenously, caused a dose-dependent decrease in arterial blood pressur e by blocking the formation of brain AngIII but not systemic AngIII. This i s corroborated by the fact that the selective APN inhibitor PC18 administer ed alone via the i.c.v. route increased arterial blood pressure. This press er response was blocked by prior treatment with the angiotensin type 1 rece ptor antagonist losartan, showing that blocking the action of APN on Ang I II metabolism leads to an increase in endogenous AngIII levels, resulting i n arterial blood pressure increase through an interaction with angiotensin type 1 receptors. These results demonstrate that AngIII is a major effector peptide of the brain RAS, exerting a tonic stimulatory control over arteri al blood pressure. Thus APA, the enzyme responsible for the formation of br ain AngIII, represents a potential central therapeutic target that justifie s the development of APA inhibitors, crossing the blood-brain barrier, as c entral anti-hypertensive agents.