Colloidal bilirubin

Most frequently, binding protein is added to the incubation mixtures as either serum or purified serum albumin. With human serum albumin, at equilibrium, the acceptor substrate will largely be protein-bound, when the bilirubin albumin molecular ratio is smaller than one (the dissociation constant of the first binding site of purified human serum albumin is approximately 7 X 10 M with 2 X 10 M for two additional binding sites) (J2). The first binding site of albumin, measured with rat serum, has a dissociation constant of about 5 X 10" M (M8). The unbound fraction will normally not exceed the very low solubility of the pigment. Addition of albumin to an alkaline solution of bilirubin, or its addition immediately after neutralization, prevents colloid formation, if the bilirubin albumin molecular ratio is smaller than one (B25). However, colloidal bilirubin, once formed, cannot be redissolved by the addition of albumin (B26). 

Some authors have used carrier-free enzymatic incubation mixtures at pH 8.0-8.3 (J5, P3, W12). In general, the final concentrations used (incubation at 37°C) were 5-10-fold higher than the solubility of bilirubin at 25°C (B25). Although solubility data at 37°C are not available, it is likely that in most instances the solubility was exceeded. It is not known whether, and to what extent, bilirubin is solubilized in an aspecific way, e.g., by dissolution in lipid membrane regions. Formation of colloidal bilirubin is possible (B25). Aging of the initial, supersaturated (B25) bilirubinate solution is expected to depend (B26) on the procedure of initial solubilization, the time elapsed between lowering the alkaline... 

Serum albumin is the most abundant protein in blood plasma. Its primary function is to control the colloidal osmotic pressure in blood, but is also important for its buffering capacity and for its ability to transport fatty acids and bilirubin, as well as xenobiotic molecules. The physiological implications of its esterase-like activity are unknown (see Sect. 3.7.5). 

The most frequent protein in the plasma, at around 45 g is albumin. Due to its high concentration, it plays a crucial role in maintaining the blood s colloid osmotic pressure and represents an important amino acid reserve for the body. Albumin has binding sites for apolar substances and therefore functions as a transport protein for long-chain fatty acids, bilirubin, drugs, and some steroid hormones and vitamins. In addition, serum albumin binds Ca "" and Mg "" ions. It is the only important plasma protein that is not glycosylated. 

At a bilirubin albumin molecular ratio below one the added binding protein will thus act as a kind of buffer, keeping the concentration of unbound substrate sufficiently low to inhibit colloid formation (B25) or precipitation onto bound bilirubin (B26), and will prevent aspecific binding to cell particulates. The binding protein can also be thought of as a reservoir providing a continuous stream of molecularly dispersed sub-... 

Brodersen, R., and Theilgaard, J., Bilirubin colloid formation in neutral aqueous solution. Scand. J. Clin. Invest. 24, 396-398 (1969). 

Binding and transport. There are four binding sites on the albumin molecule, with varying specificity for different snbstances (snch as fatty acids and bilirubin). Competitive binding of drugs may occur at the same site, with displacement of the originally bound substance. Maintenance of colloid osmotic pressure. Albumin is responsible for 75-80 % of osmotic pressure. 

ALBUMIN Albumin is a normal plasma protein with two main functions to maintain the colloid osmotic pressure in blood plasma and bind and transport low molecular substances e.g. bilirubin, fatty acids, hormones and certain drugs. Administration of 1 g albumin increases the circulating plasma volume by about 18 ml. At normal serum albumin levels, 40-50 g/1 albumin is responsible for about 80% of the colloid osmotic pressure. 

The hepatocytes are the most active sites of protein synthesis, and albumin plays important roles in the transport of bilirubin, anions, fatty acids, several hormones, and xenobiotics. Albumin also is important in determining the colloidal osmotic pressure of plasma and other body fluids. Other proteins synthesized in the liver include the acute-phase response proteins, complement proteins, and the coagulation cascade proteins (see Chapter 8). The levels of plasma proteins reflect the balance between the rates of synthesis, utilization, and degradation. The liver also plays an important role in the metabolism of cholesterol and lipoproteins (see Chapter 9). 

Baydemir G, Bereli N, Andac M, Say R, Galaev lY, Denizli A (2009) Bilirubin recognition via molecularly imprinted supermacroporous cryogeis. Colloid Surf 68 33-38... 

A fraction of the intestinaly excreted polymer may be transported into the intestinal lumen with the bile. Bile is produced by the liver parenchymal cells (hepatocytes) by active secretion in the sense that the hepatocytes transform and transport blood components, bile acids and bilirubin into the bile canaliculi. There are some indications that macromolecules and even particles, e.g. colloidal mercuric sulfite, colloidal Ag, can be eliminated from blood in the bile although this elimination is slow. De... 

A protein having a molecular weight of approximately 65 000. It has a transport role in the blood since it can reversibly bind long chain fatty acids, bilirubin, calcium and certain hormones, e.g. thyroxine and cortisol. In addition to its transport functions it can serve as a reserve store of protein and contributes significantly to plasma colloidal osmotic pressure. Two congenital disorders of albumin synthesis have been described, analbuminaemia when there is deficient synthesis, and bisalbuminaemia when two types of albumin occur. 

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