Hydrodynamic radii versus molecular weights in clearance studies of urine and cerebrospinal fluid

The protein permeabilities of the blood-cerebrospinal-fluid barrier and the blood-urine barrier are compared. In both instances the permeation rates of proteins can be better correlated to hydrodynamic radii than to molecular weights. The decreasing clearance rates of ceruloplasmin, IgG, and IgA reflect the hydrodynamic heterogeneity of these proteins, which have similar weights. The IgM clearance rate appears too high, possibly owing to monomer IgM, other IgM fragments, selective transport, or active immunity, so its inclusion is not recommended in clearance studies.