A mean distance of more than 100 Å separates the surfaces of lipoproteins and rat erythrocytes

The distribution volumes of125l‐labelled IgG (γ‐immunoglobulin).32P‐labelled VLDL (very light density lipoproteina) (d< l 016). or32l‐labelled HDL (high density lipoproteins) (1.11 < d <1.16) and [3H] inulin in packed erythrocytes from rats were measured by the dilution technique. in two sudsequent steps using approximately 5‐fold dilutions of the trapped extracellular volume. initially louded with the tracers. The total cell area. the Stoke radius of inulin and the distribution volume of inulin together define in each experiment a surface of reference not necessarily identical with the cell surface. Centrifugation of a standard cell concentration 10000xg tor 30. 60 or 90 s produced fractional distribution volumes of inulin between 85 and 37mU/g. The discoid form of the cells was preserved in the expermiental medium and the total cell area of the packed cells was calculated from the volume to surface area ratio eatimated to 6024 A. The minimum average distance between the surface of reference and the protein or a lipoprotein particle was calculated on the basia of the difference in distribution volumea of each of these substances and inulin. The distance was 138±9.5 A (n= 8) (mean ±SD) for IgG independent of the bize of the inulin space. For VLDL and HDL the calculated distances varied significantly with the fractional inulin space of an average thickness less than 140 A in excess of the maximum levels of 299±20 A (n=13) and 191±15 A (n=12) for the lipoproteins respectively. Subtractions of the Stoke radii of the protein and the particles from these distances indicate that the surfaces of IgG. VLDI, and HDL are spaced from the surface of reference by about 87 A. 100 A and 125 A respectively. The distances to the true cell surface are greater in so far as the inulin concentration of the pericellular space is diminished by the fibers of the glycocalyx. Together with lack of affinity of the particles to the cell surface the data make an exchange of molecules by contact unlikely.