Problems of using radical scavengers in biological systems

When a radical R, is formed in a homogeneous solution, where it reacts with a target molecule T, the addition of a scavenger S, which competes with T for R, will protect the target. If the damage by R is exogeneous, the addition of a scavenger S/sub i/, will protect the cells. If however the damage by R is endogeneous, the situation is different. In this case R is formed inside the cell and reacts with the target there, and S/sub i/ has to penetrate into the cell in order to protect it. The extent of the protection, since S/sub i/ in the cell and S/sub i/ in the suspension may be different will no longer be proportional to the product k/sub R+S/sup (s)//. In these cases protection by radical scavengers are expected to be observed at high concentrations of S/sub i/ and will not be always proportional to the product of k/sub R+S/ by the concentration of S/sub i/ in the suspension