title: Radioligand binding reveals chymase as the predominant enzyme for mediating tissue conversion of angiotensin I in the normal human heart.
abstract: 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, (125)I-[Sar(1),Ile(8)]angiotensin II bound with sub-nanomolar affinity, with a corresponding K(D) of 0.42+/-0.09 nM, a B(max) of 11.2+/-2.3 fmol.mg(-1) protein and a Hill slope of 1.04+/-0.04. The rank order of inhibitory potency of competing ligands for the (125)I-[Sar(1),Ile(8)]angiotensin II binding site was CGP42112>angiotensin II> or =angiotensin III=angiotensin I>losartan. The angiotensin type II (AT(2)) receptor predominated in the human left ventricle over the angiotensin type I (AT(1)) receptor, with an approximate AT(1)/AT(2) receptor ratio of 35:65. No specific (125)I-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 AT(1) receptor antagonists, the left ventricle of the human heart expresses mainly AT(2) receptors and the tissue-specific conversion of angiotensin I occurs predominantly via chymase rather than ACE.
authors.affiliation: Clinical Pharmacology Unit, University of Cambridge, Level 6, Centre for Clinical Investigations, Box 110, Addenbrooke's Hospital, Cambridge CB2 2QQ, U.K.
authors.forename: Sidath D
authors.initials: SD
authors.lastname: Katugampola
authors.forename: Anthony P
authors.initials: AP
authors.lastname: Davenport
mesh_terms: D000328:Adult; D000803:Angiotensin I / Q000378:metabolism*; D000804:Angiotensin II / Q000502:physiology; D000805:Angiotensin III / Q000502:physiology; D000806:Angiotensin-Converting Enzyme Inhibitors / Q000494:pharmacology; D053818:Chymases; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006352:Heart Ventricles / Q000201:enzymology*; D006801:Humans; D008297:Male; D009842:Oligopeptides / Q000494:pharmacology; D007703:Peptidyl-Dipeptidase A / Q000032:analysis / Q000502:physiology; D011869:Radioligand Assay / Q000379:methods*; D012697:Serine Endopeptidases / Q000032:analysis* / Q000502:physiology; D018709:Statistics, Nonparametric
chemical_list: D000806:Angiotensin-Converting Enzyme Inhibitors; D004791:Enzyme Inhibitors; D009842:Oligopeptides; D000804:Angiotensin II; D000805:Angiotensin III; D000803:Angiotensin I; C002101:chymostatin; D007703:Peptidyl-Dipeptidase A; D012697:Serine Endopeptidases; D053818:Chymases