英文文献关于茜素红定量中的一点困惑，请大家帮忙

最近我打算做茜素红定量，大体方法是用茜素红染钙结节以后，用CPC把茜素红萃取出来，然后测吸光值对比茜素红含量，但是测得茜素红含量要除以蛋白总量，我的问题是：如何提取总蛋白？我看了好多好多文献做这个茜素红定量，但是没有一个提到详细的方法，我把英文文献的material and method 部分放在这里，有心人士帮我分析下，感激不尽：
Alizarin red S staining
To determine the degree of mineralization in chondrocyte cultures, cultures
were stained with alizarin red S. After washing, chondrocyte cultures
were fixed with 70% ethanol for 10 min, and then stained with 0.5% alizarin
red S in H2O, pH 4.0, for 5 min at room temperature. After staining,
cultures were washed three times with H2O followed by 70% ethanol. To
quantify matrix mineralization, the alizarin red S–stained cultures were incubated
with 100 mM cetylpyridinium chloride for 1 h to solubilize and
release calcium-bound alizarin red into solution (Johnson et al., 2001).
The absorbance of the released alizarin red S was measured at 570 nm using
a spectrophotometer. Data are expressed as units of alizarin red S released
per milligram of protein in each culture.
最关键的最后一句我没有理解，大家能给我解释下他是怎么做的吗？