Katugampola Sidath D, Davenport Anthony P
Clinical Pharmacology Unit, University of Cambridge, Level 6, Centre for Clinical Investigations, Box 110, Addenbrooke's Hospital, Cambridge CB2 2QQ, U.K.
Clin Sci (Lond). 2002 Jan;102(1):15-21.
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.
我们研究了血管紧张素受体的结合特性，并利用该测定法确定了能够在人左心室中转化血管紧张素I的主要酶。在人左心室匀浆中，(125)I-[Sar(1),Ile(8)]血管紧张素II以亚纳摩尔亲和力结合，相应的解离常数（KD）为0.42±0.09 nM，最大结合容量（Bmax）为11.2±2.3 fmol·mg(-1)蛋白质，希尔系数为1.04±0.04。竞争配体对(125)I-[Sar(1),Ile(8)]血管紧张素II结合位点的抑制效力排序为：CGP42112＞血管紧张素II≥血管紧张素III =血管紧张素I＞氯沙坦。血管紧张素II型（AT(2)）受体在人左心室中比血管紧张素I型（AT(1)）受体占优势，AT(1)/AT(2)受体比例约为35:65。在人左心室中未检测到特异性的(125)I-血管紧张素IV结合位点。使用竞争性放射性配体结合测定法，我们能够证明糜酶/组织蛋白酶G酶抑制剂抑肽酶在抑制人左心室中血管紧张素I的转化方面比血管紧张素转换酶（ACE）抑制剂卡托普利更有效。抗蛋白酶（一种组织蛋白酶G而非糜酶的抑制剂）对血管紧张素I的转化没有影响，这表明大部分转化是由糜酶介导的。因此，尽管目前用于肾素-血管紧张素系统的疗法主要集中在ACE抑制剂和AT(1)受体拮抗剂上，但人类心脏的左心室主要表达AT(2)受体，血管紧张素I的组织特异性转化主要通过糜酶而非ACE发生。
