Interactions of unconjugated bilirubin with vesicles, cyclodextrins and micelles: New modeling and the role of high pKa values

摘要

Background: Unconjugated bilirubin (UCB) is an unstable substance with very low aqueous solubility. Its aqueous pKavalues affect many of its interactions, particularly their pH-dependence. A companion paper shows that only our priorsolvent partition studies, leading to pKa values of 8.12 and 8.44, met all essential requirements for valid pKadeterminations. Other published values, generally lower, some below 5.0, were shown to be invalid. The present workwas designed to derive suitable models for interpreting published data on the pH-dependent binding of UCB with fouragents, mentioned below, chosen because they are not, themselves, sensitive to changes in the pH range 4-10, and thedata, mainly spectrometric, were of reasonable quality.Results: These analyses indicated that the high pKa values, dianion dimerization constant and solubilities of UCB atvarious pH values, derived from our partition studies, along with literature-derived pH- and time-dependentsupersaturation effects, were essential for constructing useful models that showed good qualitative, and sometimesquantitative, fits with the data. In contrast, published pKa values below 5.0 were highly incompatible with the data forall systems considered. The primary species of bound UCB in our models were: undissociated diacid forphosphatidylcholine, dianion for dodecyl maltoside micelles and cyclodextrins, and both monoanions and dianion forsodium taurocholate. The resulting binding versus pH profiles differed strikingly from each other.Conclusions: The insights derived from these analyses should be helpful to explore and interpret UCB binding to morecomplex, pH-sensitive, physiological moieties, such as proteins or membranes, in order to understand its functions.