IC50, the half maximal inhibitory concentration, represents the concentration of an inhibitor that is required for 50% inhibition of things like an enzyme, a cell, a cell receptor or a microorganism.
It's commonly used as a measure of drug effectiveness. Another measure of drug efficacy is EC50 which represents the concentration of a compound that is required to obtain 50% of the maximum effect. (Ex. EC100 would be the minimum concentration of a compound to obtain 100% of its effect).
它的值取决于实验所用的酶和底物浓度，当酶的浓度固定时，IC50值与Ki、Km和底物浓度[S]呈如下关系：
根据该方程式，当底物浓度大于Km值时，IC50值高于Ki值，尤其当底物浓度较高时，Ki值偏低更加明显。
竞争性抑制作用Ki的求解方法
①从双倒数图计算出表观米氏常数Kmapp，然后代入Kmapp=Km(1+[I]/Ki)可计算出Ki。
②从双倒数图求得各I浓度下的Kmapp值对相应[I]再作图，从再制图的纵截距可直接测得Km，从其横截距可直接测得Ki。
③从不同固定[I]下所作一簇直线的斜率1/S对相应[I]再制图，其纵截距为Km/Vmax, 斜率为Km/VmaxKi，横截距即为-Ki。
④Dixon作图法求Ki值
