Role of apolipoprotein D in the transport of bilirubin in plasma

Abstract

Apolipoprotein D (apo D) is a 30-kDa glycoprotein of unknown function that is associated with high-density lipoproteins (HDL). Because unconjugated bilirubin has been shown to bind apo D with a 0. 8:1 stoichiometry, we examined the contribution of this protein to transport of bilirubin in human plasma. Density gradient centrifugation analysis using physiological concentrations of [(14)C]bilirubin reveals that 9% of unconjugated bilirubin is associated with HDL, with the remaining pigment bound primarily to serum proteins (i.e., albumin). The percentage of total plasma bilirubin bound to HDL was found to increase proportionally with bilirubin concentration. Affinity of human apo D for bilirubin was determined by steady-state fluorescence quenching, with Scatchard analysis demonstrating a single binding site for unconjugated bilirubin with an affinity constant (K(a)) of approximately 3 x 10(7) M(-1). Incorporation of apo D into phosphatidylcholine vesicles had no effect on K(a), suggesting that a lipid environment does not alter the affinity of the protein for bilirubin. Using stopped-flow techniques, the first-order rate constant for bilirubin dissociation from apo D was measured at 5.4 s(-1) (half-time = 129 ms). Our findings indicate that HDL is the principal nonalbumin carrier of bilirubin in human plasma and further support the proposition that the affinity of HDL for bilirubin is primarily the result of binding to apo D.

PGE2-regulated wnt signaling and N -acetylcysteine are synergistically hepatoprotective in zebrafish acetaminophen injury

“…Prior proteomic studies in the zebrafish have focused on the embryo and adult liver (13,29). Our analysis revealed an absence of albumin, the major mammalian serum transport protein, consistent with recent genome analyses (30); apolipoproteins and vitellogenin may serve as alternative carriers for hydrophobic compounds in zebrafish blood (31). We found distinct serum protein patterns of response to APAP and concordant transcriptional gene regulation.…”

Reassessment of the Unbound Concentrations of Unconjugated Bilirubin in Relation to Neurotoxicity In Vitro

“…There are, however, important differences between plasma or serum and in vitro systems that limit comparisons between his study and ours. Plasma contains additional proteins that bind UCB, such as apo D (41), so that B F levels in plasma are lower than those in solutions containing the same concentration of purified albumin (42). On the other hand, FFA and other substances not present in defined solutions containing delipidated albumin may inhibit the binding of UCB to albumin (7).…”

The hemolymph proteome of Anopheles gambiae

“…It is approximately 30% identical to a range of proteins from other animals, including nervous system lipocalin-like proteins from B. mori (bombyrin, Sakai et al, 2001) and Locusta migratoria (lazarillo, Sanchez et al, 1995), Manduca sexta insecticyanin, and several apolipoprotein Ds from chordates. Interestingly, apolipoprotein D and insecticyanin both bind bilin pigments resulting from the catabolism of heme (bilirubin and biliverdin, respectively) with high affinity, and both occur in plasma (Goessling and Zucker, 2000;Goodman et al, 1985). Perhaps LLP-1 plays a similar role in A. gambiae in relation to blood feeding.…”