We investigated the binding characteristics of angiotensin receptors and used this assay to determine the predominant enzyme capable of converting angiotensin I in the human left ventricle. In homogenates of human left ventricle, 125I-[Sar1,Ile8]angiotensin II bound with sub-nanomolar affinity, with a corresponding KD of 0.42±0.09nM, a Bmax of 11.2±2.3fmolċmg-1 protein and a Hill slope of 1.04±0.04. The rank order of inhibitory potency of competing ligands for the 125I-[Sar1,Ile8]angiotensin II binding site was CGP42112 > angiotensin II⩾ angiotensin III = angiotensin I > losartan. The angiotensin type II (AT2) receptor predominated in the human left ventricle over the angiotensin type I (AT1) receptor, with an approximate AT1/AT2 receptor ratio of 35:65. No specific 125I-angiotensin IV binding sites could be detected in the human left ventricle. Using competitive radioligand binding assays, we were able to demonstrate that the chymase/cathepsin G enzyme inhibitor chymostatin was more potent than the angiotensin-converting enzyme (ACE) inhibitor captopril at inhibiting the conversion of angiotensin I in the human left ventricle. Aprotonin (an inhibitor of cathepsin G but of not chymase) had no effect on angiotensin I conversion, suggesting that the majority of the conversion was mediated by chymase. Thus, although the current therapies used for the renin-angiotensin system have focused on ACE inhibitors and AT1 receptor antagonists, the left ventricle of the human heart expresses mainly AT2 receptors and the tissue-specific conversion of angiotensin I occurs predominantly via chymase rather than ACE.
Identification of a highly specific chymase as the major angiotensin II-forming enzyme in the human heart.
