Dialyzability, Protein Binding, and Renal Excretion of Plasma Conjugated Bilirubin.

Conjugated bilirubin is excreted in the urine, but unconjugated bilirubin is generally believed not to be excreted by the kidneys. The basis for this difference has not been resolved by past studies. Hoover and Blankenhorn reported that bilirubin was dialyzable only from the plasma of patients who had bilirubinuria, and that it was not dialyzable from the plasma of patients with acholuric jaundice (3). Most subsequent investigators, however, have maintained that neither conjugated nor unconjugated bilirubin was dialyzable or ultrafilterable from plasma (4-7). This led to the hypothesis that conjugated bilirubin was probably excreted in the urine by a process of renal tubular secretion (8). Studies that were performed in jaundiced dogs in this laboratory gave results consistent with a process of glomerular filtration of conjugated bilirubin (9). We therefore re-examined the question of the dialyzability of plasma bilirubin. A small fraction of plasma conjugated bilirubin in jaundiced humans and dogs was found to be dialyzable, and hence available for urinary excretion by glomerular filtration. The studies indicated that this dialyzable fraction was bound to low molecular weight carrier(s), which migrated in the alpha and beta globulin zones on paper electrophoresis and which sedimented on ultracentrifugation.