Endothelin-2 (ET-2) is a 21-amino acid vasoactive peptide that belongs to the endothelin family, which also includes ET-1 and ET-3. These peptides are produced by various cell types and bind to two G-protein-coupled transmembrane receptors, the ET-A receptor (ET-RA) and the ET-B receptor (ET-RB). ET-2 is encoded by a distinct, independently expressed gene and has been shown to be expressed in several human tumors, including breast carcinomas. In infiltrating ductal carcinomas of the breast, ET-2 and ET-RB mRNA are frequently detected, whereas they are largely absent in normal breast tissue. ET-2 expression is regulated by hypoxia in a hypoxia-inducible factor 1 (HIF-1)-dependent manner, with mRNA levels increasing within hours of hypoxic exposure in breast tumor cell lines.

In hypoxic conditions, ET-2 acts as an autocrine survival factor for breast tumor cells. The peptide is released by tumor cells and signals primarily through the ET-RB receptor to protect cells from hypoxia-induced apoptosis. This protective effect is specific to hypoxic stress, as ET-2 or its receptor antagonists have no significant effect on cell growth or survival under normoxic conditions. The upregulation of ET-2 and its receptors during hypoxia represents a cellular mechanism that allows tumor cells to withstand low-oxygen environments, and this process is colocalized with hypoxic regions within tumors.

• Grimshaw MJ, Naylor S, Balkwill FR. Endothelin-2 is a hypoxia-induced autocrine survival factor for breast tumor cells. Mol Cancer Ther. 2002 Dec;1(14):1273-81. PMID: 12516960.

三氟乙酸（Trifluoroacetic acid，简称 TFA）是高效液相色谱（简称 HPLC）纯化过程中常用的反离子。TFA 的存在可能会对肽的净重量、外观及溶解度产生影响，具体如下：

对净重量的影响：TFA 盐会增加产物的总质量。在大多数情况下，肽的含量占总重量的 80% 以上，剩余部分则为 TFA。

对溶解度的影响：TFA 盐通常能提高肽在水溶液中的溶解度。

在生物检测中的影响：对于大多数标准的体外检测，残留的 TFA 水平一般不会造成干扰。但需注意，在高灵敏度的细胞研究或生化研究中，需关注其存在可能带来的影响。

干品多肽的重量中不仅仅包含多肽，还包含有一些非肽的组份，如水、被吸收的溶剂、配位离子和盐等。肽的净含量是指肽在其中的重量百分比，这个百分比的数值范围很大，可能从50%到90%，取决于纯度、序列以及合成和纯化的方法，不要将肽的净含量和肽的纯度混为一谈，他们是两个完全不同的概念。纯度通常是通过HPLC测定的。纯度定义的是多肽样品中含正确序列的组分的百分比，而肽的净含量是指样品中肽类物质相对于总物质所占的百分比，肽的净含量通常是氨基酸组分分析或紫外分光法测定的，这个信息主要是在一些对肽的浓度很敏感的实验中，对计算肽的浓度是很重要的。