Effect of linear reinsertion of receptor on the distribution of receptors around coated pits.

We consider a linear receptor reinsertion step in our kinetic model of the primary steps occurring in receptor-mediated endocytosis. In contrast with our previous zeroth order receptor reinsertion assumption, here we consider a first order process, and we study the effect of receptor diffusion on the trapping rate constant (k+) and the radial distribution of receptors around coated pits (grp). Using experimental data for low density lipoproteins (LDL) receptors on fibroblast cells, we find that the trapping of receptors by coated pits is diffusion-controlled for any value of the escaping rate constant (k-). This result is significantly different from our previous findings. In fact, for a zeroth order process, we find that either diffusion has no effect on k+ or, at the most, receptor trapping is 84% diffusion-controlled. Moreover, we find values for the receptor reinsertion rate constant (kappa), which range between 15% of the pit's invagination rate constant, lambda, and three halves lambda. In addition, the ratio kappa/lambda is equal to the ratio of the concentration of receptors in pits with respect to the internalized receptors. Comparison between the experimental radial distribution of receptors around pits and the theoretical should provide an indication of the diffusion effect on k+.