Conformational “Fingerprint” of the Angiotensin-Converting Enzyme

The angiotensin-converting enzyme (ACE) is a zinc-dependent metalloproteinase widely occurring in the organism; it metabolizes many peptides and plays a key role in blood pressure regulation and vascular remodeling. This enzyme is expressed as a type-1 membrane glycoprotein on the surface of endothelial and epithelial cells, but is also found in a soluble form in biological fluids. In this study, we used purified ACE from lungs, which is mainly produced by endothelial cells of lung capillaries; ACE from heart, produced by endothelial heart cells and, probably, by myofibroblasts; and ACE from seminal fluid, produced by the epithelial cells of the prostate and epididymis. The pattern of binding of a set of 17 mAbs to different conformational epitopes on the surface of two domains of the human ACE significantly differed for ACEs from different organs. This pattern (the conformational “fingerprint” of ACE) reflects the local conformation of the surface of a particular ACE. The differences in the conformational fingerprints of ACEs expressed by different cell types, or even by similar cells but in different organs, can be explained by the posttranslational modification of ACE protein in these organs and, primarily, different glycosylation of N-glycosylation sites Asn25, Asn117, Asn289, Asn666, Asn685, and Asn731. The mass spectrometry of tryptic hydrolyzates of ACEs isolated from different human organs made it possible to reveal, in the composition of different ACEs, N-glycosylation sites that are really occupied by glycans, namely, Asn in positions 82, 117, 416, 648, 666, 685, and 731 in ACE from seminal fluid; Asn in positions 117, 648, 666, and 685 in ACE from lungs; and Asn in positions 117, 480, 666, and 685 in ACE from heart. Differences in the plausible structures of glycans in ACE, in particular, at the Asn666 N-glycosylation site were demonstrated, which can explain the differences in the efficiency of binding of mAbs to ACE from different organs.