**5.2 Biosynthesis of endothelin-1**

ET-1 peptide is most abundant and widely expressed of the three isoforms such as ET-1, ET-2, and ET-3 [22]. ET-2 and ET-3 exhibit two and six different amino acids, respectively, compared to ET-1. ET-1 has a molecular weight of 2492, a hydrophobic carboxyl terminus, and two intramolecular disulfide bonds near the amino terminus [22]. It is the only isoform thought to be constitutively released from endothelial cells and is synthesized as the result of a series of proteolytic cleavages of the initial gene product – the preproendothelin – an inactive precursor 212 amino acids long. A 17-aa leader sequence targets preproET-1 to the endoplasmic reticulum where it enters the secretory pathway [23]. The precursor peptide is processed by furin-like proteases to biologically inert intermediates pro-endothelin1 and the 38-aa "big ET-1." Endothelin-converting enzyme (ECE) cleaves the bond between Trp 21 and Val22 [24–26] to generate the mature 21-aa active ET-1 peptide (**Figure 1**).

#### **Figure 1.**

*Schematic representation of endothelin-1 and its biosynthesis and the localization of endothelin receptor subtypes on vascular smooth muscle cells and endothelial cells. Abbreviations: ET-1, endothelin-1; ETR-A, endothelin receptor A; ETR-B, endothelin receptor B; ECE, endothelin-converting enzyme. Figure created by using the Bioservier Medical Art.*

ET-1 is synthesized by a dual pathway being released continuously by the secretory vesicles of the constitutive pathway to maintain the vascular tone [27]. They are also stored in Weibel-Palade granules of endothelial cells and released by exocytosis and degranulation in a regulated manner when exposed to pathophysiological stimuli [28].

Under physiological conditions, blood flow appears to regulate ET-1 synthesis and release via the "shear stress receptors" on endothelial cells. This endothelin synthesis is activated in response to major cardiovascular risk factors such as

hyperglycemia [29, 30], hypercholesterolemia [31], arterial hypertension, estrogen deficiency [32], and aging [25], as well as by biochemical and mechanical stimuli.
