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 cont rast with our previous zeroth order receptor reinsertion assumption, h ere we consider a first order process, and we study the effect of rece ptor diffusion on the trapping rate constant (k+) and the radial distr ibution of receptors around coated pits (g(rp)). Using experimental da ta for low density lipoproteins (LDL) receptors on fibroblast cells, w e find that the trapping of receptors by coated pits is diffusion-cont rolled for any value of the escaping rate constant (k-). This result i s significantly different from our previous findings. In fact, for a z eroth order process, we find that either diffusion has no effect on k or, at the most, receptor trapping is 84% diffusion-controlled. Moreo ver, we find values for the receptor reinsertion rate constant (kappa) , which range between 15% of the pit's invagination Tate constant, lam bda, and three halves lambda. In addition, the ratio kappa/angstrom is equal to the ratio of the concentration of receptors in pits with res pect to the internalized receptors. Comparison between the experimenta l radial distribution of receptors around pits and the theoretical sho uld provide an indication of the diffusion effect on k+.