Albumin, bilirubin complex

Figure 22-3. Transport and hepatic metabolism of bilirubin. Bilirubin that is produced in phagocytes is transported to liver as an albumin-bilirubin complex. Uptake into the hepatocytes takes place in liver sinusoids. Within the hepatocyte, bilirubin is transported to the endoplasmic reticulum (microsomes) bound to glutathione S-transferase (GST). Bilirubin is made water soluble by addition of one or two glucuronic acid moieties obtained from UPD-glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. The product, conjugated bilirubin, is transported across the bile canalicular membrane for secretion into the biliary system, with subsequent movement into the intestines.

Since binding of bilirubin to albumin is usually reversible, a small amount of free bilirubin is present in plasma in equilibrium with albumin-bound bilirubin. It is probably this free bilirubin that is taken up at a rate determined by its plasma concentration. As this free bilirubin concentration decreases, more bilirubin is released from albumin and becomes available for uptake. Alternatively, the albumin-bilirubin complex may bind to specific hepa-tocyte plasma membrane receptors, and thereby bilirubin is released to enter the cell. Both models are consistent... 

The CD of bilirubin-complexes with serum albumin from a variety of species are very different under identical conditions. In several complexes, the observed CD spectrum is far more complicated than what is observed in the case of HSA. Other rotational strength mechanisms, such as coupling with protein groups, were suggested as additional sources of optical activity [239]. 

While Bennhold s first studies were made on whole serum, he later proceeded to study serum fractions, and showed that the cbaracteristic dye binding in most cases was due almost entirely to the albumin fraction. These conclusions were further extended by a long series of studies on the cataphoresis of serum proteins and protein fractions, using the type of apparatus developed by Michaelis. The attachment of these various dyes to the proteins was clearly revealed, and the color of the dye-protein complex permitted the observation of a well-defined boundary. Definite limits to the binding capacity of serum albumin for dye were observed in these studies. Above a certain critical concentration free dye was recognizable in solution and moved in the electric field with a velocity different from that of the albumin-dye complex. Bennbold also showed that bilirubin, when added to serum, was quantitatively bound to the albumin, up to a certain limited binding power. 

The bilirubin that is produced in phagocytic cells from degradation of hemoglobin represents the majority of the bilirubin that is produced and must be eliminated. This initially requires transport of bilirubin from the phagocytic cells to the liver. Normally, bilirubin is secreted from phagocytic cells and complexed with albumin for transport to the liver. It is essential that bilirubin is transported through the circulation bound to albumin. The toxicity of... 

Bilirubin - an apolar, water-insoluble lipophile substance - is potentially toxic. It is bound to serum albumin and transported to the sinusoidal membrane of the liver cell as a bilirubin-albumin complex, (s. fig. 3.1) The binding capacity of albumin is exceeded only at a serum bilirubin concentration of >4—5 mg/dl. In the case of decreased albumin binding (e. g. in acidosis) or oversaturated binding capacity, there is a danger of toxic cell damage due to the diffusion of unbound bilirubin into the cells (in some cases accompanied by kernicterus). Neonates and premature babies are at particular risk because of their immature blood-cerebrospinal fluid barrier. Albumin-bound bilirubin can function as an antioxidant to intercept free radicals and/or O2 radicals. (93) (s. tab. 3.25)... 

EN45 lhara, H., Nakamura, H., Aoki, Y., Aoki, T. and Yoshida, M. (1991). Effects of serum-isolated vs synthetic bilirubin-albumin complexes on dye-binding methods for estimating serum albumin. Clin. Chem. 37, 1269-1272. 

The complex formation of bilirubin with human serum albumin was investigated by Sinclair et al. 72) using 347 nm ruby laser flash photolysis technique. A high bilirubin level is found in new bom babies who suffer from jaundice (neonatal hyperbilirubinemia)73). Phototherapy has been found to be suitable for lowering the bilirubin level. In order to understand the mechanism of the phototherapy, investigation into the photophysics of bilirubin is essential. It is strongly bound to human serum albumin, lipids and cell membranes. 

Other causes of altered mobility. Increased mobility of albumin occurs when it is bound to penicillin or salicylates or to greater-than-normal amounts of bilirubin or fatty acids. Mercaptaibumin dimers or complexes with AAT migrate cathodal to monomeric albumin. Decreased mobility of AAT occurs when it binds thiol groups of IgA or Bence Jones protein. 

Bilirubin is a lipid-soluble metabolite of hemoglobin that is transported through the bloodstream as an albumin complex to the liver where it is esterified for excretion in urine. It plays an important role in the pathology of many diseases. Its Raman spectrum has been difficult to observe because of its sensihvity to photoisomerization and decomposition. However, under condihons of SERS on Ag electrodes this is suppressed allowing potenhal-dependent spectra to be observed in its free form as well as complexed with cyclodextrins and albumin [327]. The band intensities are higher at more posihve potenhals implicahng adsorption of anionic species on the electrode. 

Bilirubin formed in extrahepatic tissues is transported to the liver for excretion in bile. Since bilirubin is virtually insoluble in aqueous media, it is transported to the liver bound noncovalently to serum albumin. The bilirubin-albumin complex increases the amount of bilirubin carried... 

Fig. 11. CD spectra of the complex bilirubin-(charcoal-treated) human serum albumin at different pH values. Temp., 27.0 0.5 °C. measured at pH 4.00 0.02 (inset) 0, measured at pH 7.3 0.1 O, measured at pH 9.7 0.3. Drawn curves at pH 4.0, 7.3 and 9.7- data calculated by computer from experimental data (points). Resolution into two Gaussian curves at pH 4.0 and 9.7, and into three Gaussians at pH 7.3. Drawn curves at pH 4.8 and 6.0 are based on experimental data only. [Reproduced from Ref. 308) where conditions are given]...

Following intravenous injection, the Tc-IDA complex is bound to plasma protein (mainly albumin) and carried to the liver (Nicholson et al. 1980). Accumulation in the liver involves the same carrier-mediated, non-sodium-dependent organic anion transport processes as for the uptake of bilirubin. In the space of Disse, the albumin- Tc-IDA conjugate is dissociated to facilitate active transport of the Tc-IDA complex into hepatocytes (Krishnamurthy and Krishnamurthy 1989). In patients with normal hepatobiliary function, maximal liver uptake is measured at 12 min ( Tc-mebrofenin, 10.9 1.9 min Tc-disofenin, 11.5 3.1 min) (Fritzberg 1986). The radioactivity is half this value within approximately 20 min. The gallbladder is well visualized 20 min postinjection. Intestinal activity appears on the average at 15-30 min. The common bile duct may be visualized after 14 min. The upper limit of normal for visualization of these structures is 1 h (Weissmann et al. 1979). 

Heme -> Biliverdin -> Bilirubin -> (passage through blood to liver as bilirubin-albumin complex) -> Bilirubin diglucuronide -> excretion. 

Bilirubin is insoluble in aqueous solutions, so complexing with albumin and gluruomc acid is essential for passage through the body. Accumulation of bilirubin in the blood leads to jaundice. 

For the construction of artificial metalloproteins, protein scaffolds should be stable, both over a wide range of pH and organic solvents, and at high temperature. In addition, crystal structures of protein scaffolds are crucial for their rational design. The proteins reported so far for the conjugation of metal complexes are listed in Fig. 1. Lysozyme (Ly) is a small enzyme that catalyzes hydrolysis of polysaccharides and is well known as a protein easily crystallized (Fig. la). Thus, lysozyme has been used as a model protein for studying interactions between metal compounds and proteins [13,14,42,43]. For example, [Ru(p-cymene)] L [Mn(CO)3l, and cisplatin are regiospecificaUy coordinated to the N = atom of His 15 in hen egg white lysozyme [14, 42, 43]. Serum albumin (SA) is one of the most abundant blood proteins, and exhibits an ability to accommodate a variety of hydrophobic compounds such as fatty acids, bilirubin, and hemin (Fig. lb). Thus, SA has been used to bind several metal complexes such as Rh(acac)(CO)2, Fe- and Mn-corroles, and Cu-phthalocyanine and the composites applied to asymmetric catalytic reactions [20, 28-30]. 

After the reduction of biliverdin, the bilirubin that is formed then undergoes a series of transport and transformation steps which ultimately lead to its excretion in the intestinal tract. From the sites of its production, bilirubin is released into the plasma where it efficiently binds to albumin, which acts as a plasma-transport system. The bilirubin-albumin complex is carried in the plasma to liver cells (hepatocytes), where the bilirubin is released from its albumin carrier protein and transported across the cell bilayer membrane into the hepatocyte. Once inside, the bilirubin is bound in the cytoplasm to anion-binding proteins such as ligandin. The latter carries the bilirubin to membrane-bound enzymes (localised in the endoplasmic reticulum of the liver cell) which catalyse the esterification of bilirubin, the ester groups (mainly p-D-glucuronoside, but also smaller amounts of fi-D-xylo-pyranosides and p-D-glucopyranosides) being transferred from their uridine diphosphate nucleotides. 

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