The Importance of Free Bilirubin Acid Salt in Bilirubin Uptake by Erythrocytes and Mitochondria

Abstract

ABSTRACI'. The binding of bilirubin to tissue was studied using adult human erythrocytes and rat liver mitochondria. Tissues were incubated with varying bilirubin-albumin molar ratios, varying albumin concentrations of a given bilirubin-albumin molar ratio, and varying pH. Bilirubin binding by tissue was reversible and stoichiometric with the concentration of the free (nonalbumin bound) bilirubin acid salt (bilirubin monovalent anion). Minimal binding of the bilirubin dianion, the predominant state of bilirubin in plasma, was also suggested. The observations support the "free bilirubin theory" where tissue and albumin compete for binding the body's bilirubin pool. Binding to tissue, however, is not determined by the free bilirubin concentration, but by the concentration of the pH dependent subfraction, the free bilirubin acid salt. Tissue binding and toxicity of bilirubin may result from the surfactant properties of the monovalent anion. (Pediatr Res 23: 443-447, 1988)

Unconjugated Bilirubin Exhibits Spontaneous Diffusion through Model Lipid Bilayers and Native Hepatocyte Membranes

“…While this finding has been construed as evidence for electrogenic uptake, it is equally consistent with a model of bilirubin transport across biologic membranes, originally proposed by Wennberg (5,45), in which unconjugated bilirubin traverses the hydrophobic membrane core as the uncharged diacid. We have shown that bilirubin induces acidification of the entrapped volume of phospholipid vesicles, supporting a mechanism whereby bilirubin anions acquire a proton(s), diffuse through the lipid bilayer, and subsequently release hydrogen ions into the vesicle interior (31).…”

“…These unique physical-chemical properties have generated controversy regarding the mechanism of bilirubin uptake by the liver. Based on the spontaneous leakage from multilamellar liposomes (4), it has been proposed that bilirubin is able to diffuse through cellular membranes (5,6). On the other hand, in vivo (7) and whole organ (8,9) studies indicate that hepatic bilirubin uptake is saturable and occurs against a concentration gradient, findings that support a protein-mediated transport mechanism.…”

New Concepts in Bilirubin and Jaundice: Report of the Third International Bilirubin Workshop, April 6–8, 1995, Trieste, Italy

“…197 A noninvasive spectroscopic device, akin to the ear oximeter, that can monitor both COHb levels lieved to be the toxic species that diffuses into the brain, 16,173 although there are dissenting views. 180 Free bilirubin levels and total bilirubin in plasma every 5 to 10 milliseconds is under development. Any infant whose CO production rates increase when plasma albumin levels decrease, when drugs are administered that displace UCB from albumin, 170 or when do not decrease during the first postnatal week must be considered to have some additional pathological hemolytic prothe proportion of H 2 B increases because of decreasing pH.…”

Bilirubin-Induced Changes in Brain Energy Metabolism after Osmotic Opening of the Blood-Brain Barrier

“…Under physiologic conditions, tissue binding of bilirubin increases in direct proportion to the hydrogen ion concentration, indicating that it is governed by a single proton addition to bilirubin (43). Assuming an acid ionization constant for the propionic groups on bilirubin, Wennberg (43) proposed that the proton addition represents conversion of the bilirubin dianion to the monovalent anion. Given suficient fluidity of the mitochondrial membrane, the monoanion might serve as a proton shuttle, altering the tightly controlled hydrogen ion gradient critical for oxidative phosphorylation (4 1).…”