ACE2, the first known human homologue of angiotensin-converting enzyme (ACE), was identified from 5' sequencing of a human heart failure ventricle cDNA library. ACE2 has an apparent signal peptide, a single metalloprotease active site, and a transmembrane domain. The metalloprotease catalytic domains of ACE2 and ACE are 42% identical, and comparison of the genomic structures indicates that the two genes arose through duplication. In contrast to the more ubiquitous ACE, ACE2 transcripts are found only in heart, kidney, and testis of 23 human tissues examined. Immunohistochemistry shows ACE2 protein predominantly in the endothelium of coronary and intrarenal vessels and in renal tubular epithelium. Active ACE2 enzyme is secreted from transfected cells by cleavage N-terminal to the transmembrane domain. Recombinant ACE2 hydrolyzes the carboxy terminal leucine from angiotensin I to generate angiotensin 1-9, which is converted to smaller angiotensin peptides by ACE in vitro and by cardiomyocytes in culture. ACE2 can also cleave des-Arg bradykinin and neurotensin but not bradykinin or 15 other vasoactive and hormonal peptides tested. ACE2 is not inhibited by lisinopril or captopril. The organ- and cell-specific expression of ACE2 and its unique cleavage of key vasoactive peptides suggest an essential role for ACE2 in the local renin-angiotensin system of the heart and kidney. The full text of this article is available at http://www. circresaha.org.

High density lipoprotein (HDL) and low density lipoprotein (LDL) are cholesterol transport particles whose plasma concentrations are directly (LDL) and inversely (HDL) correlated with risk for atherosclerosis. LDL catabolism involves cellular uptake and degradation of the entire particle by a well-characterized receptor. HDL, in contrast, selectively delivers its cholesterol, but not protein, to cells by unknown receptors. Here it is shown that the class B scavenger receptor SR-BI is an HDL receptor. SR-BI binds HDL with high affinity, is expressed primarily in liver and nonplacental steroidogenic tissues, and mediates selective cholesterol uptake by a mechanism distinct from the classic LDL receptor pathway.

Human angiotensin-converting enzyme-related carboxypeptidase (ACE2) is a zinc metalloprotease whose closest homolog is angiotensin I-converting enzyme. To begin to elucidate the physiological role of ACE2, ACE2 was purified, and its catalytic activity was characterized. ACE2 proteolytic activity has a pH optimum of 6.5 and is enhanced by monovalent anions, which is consistent with the activity of ACE. ACE2 activity is increased ϳ10-fold by Cl ؊ and F ؊ but is unaffected by Br ؊ . ACE2 was screened for hydrolytic activity against a panel of 126 biological peptides, using liquid chromatographymass spectrometry detection. Eleven of the peptides were hydrolyzed by ACE2, and in each case, the proteolytic activity resulted in removal of the C-terminal residue only.

The carboxypeptidase ACE2 is a homologue of angiotensin-converting enzyme (ACE). To clarify the physiological roles of ACE2, we generated mice with targeted disruption of the Ace2 gene. ACE2-deficient mice were viable, fertile, and lacked any gross structural abnormalities. We found normal cardiac dimensions and function in ACE2-deficient animals with mixed or inbred genetic backgrounds. On the C57BL/6 background, ACE2 deficiency was associated with a modest increase in blood pressure, whereas the absence of ACE2 had no effect on baseline blood pressures in 129/SvEv mice. After acute Ang II infusion, plasma concentrations of Ang II increased almost 3-fold higher in ACE2-deficient mice than in controls. In a model of Ang II-dependent hypertension, blood pressures were substantially higher in the ACE2-deficient mice than in WT. Severe hypertension in ACE2-deficient mice was associated with exaggerated accumulation of Ang II in the kidney, as determined by MALDI-TOF mass spectrometry. Although the absence of functional ACE2 causes enhanced susceptibility to Ang II-induced hypertension, we found no evidence for a role of ACE2 in the regulation of cardiac structure or function. Our data suggest that ACE2 is a functional component of the renin-angiotensin system, metabolizing Ang II and thereby contributing to regulation of blood pressure.

The specific recognition of anionic phospholipids in the outer leaflets of cell membranes and lipoproteins by cell surface receptors may play an important role in a variety of physiologic and pathophysiologic processes (e.g. recognition of damaged or senescent cells by the reticuloendothelial system or lipoprotein homeostasis). Several investigators have described anionic phospholipid binding to cells, and phosphatidylserine (PS) binding to a partially purified approximately 95-kDa membrane protein has recently been reported (Sambrano, G.R., and Steinberg, D. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 1396-1400). Using both direct binding and ligand competition assays in transfected cells, we have found that two class B scavenger receptors, SR-BI and CD36, can tightly bind PS and phosphatidylinositol (PI)-containing liposomes (Kd for PS liposome binding to SR-BI is approximately 15 micrograms phospholipid/ml or 0.18 nM (mol PS liposomes/l), but not phosphatidylcholine, phosphatidylethanolamine, or sphingomyelin liposomes. PS and PI liposomes, but not the others, could effectively compete with PS liposomes and modified or native lipoproteins for binding to these receptors. Phosphatidic acid, another anionic phospholipid, could also compete, but was not as effective as PS or PI. Class B scavenger receptors are the first molecularly well-defined, specific cell surface receptors for anionic phospholipids to be described.