Bilirubin binding capacity

W5. Watson, D., Bilirubin-binding capacity of blood plasma in relation to foetal erythroblastosis. Australia and New Zealand J. Obstel. Gynaecol. 6, 121-124 (1964). 

Albumin is a major transport facilitator of hydrophobic compounds which would otherwise disrupt cellular membranes. These compounds include free fatty acids and bilirubin as well as hormones such as cortisol, aldosterone, and thyroxine when these materials have exceeded the capacity of proteins normally associated with them. Albumin also binds ions, including toxic heavy metals and metals such as copper and zinc which are essential for normal physiological functioning but may be toxic in quantities in excess of their binding capacity for their carrier proteins. Binding of protons is the basis for the buffering capacity of albumin. 

The great binding capacity of the tissues for bile pigments can be appreciated if one studies the time taken for patients to lose their jaundice when an obstruction has been surgically relieved. Rundle et al. (R2) calculated that the serum bilirubin falls exponentially so that it reaches its half-value in 17.4 days. 

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)... 

Purpose and need Onsite assessment of free bilirubin and its binding capacity with albumin... 

Capillary electrophoresis CE-frontal analysis to determine free bilirubin and its albumin-binding capacity... 

If an anemia is detected further studies including a careful examination of the peripheral smear, reticulocyte count, stool for occult blood, serum iron, total iron binding capacity, bilirubin, and, if appropriate, vitamin B12 and folate may be of value in attempting to identify the cause of the anemia. 

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. 

Delayed uptake of bilirubin has been described in mutant Southdown sheep (CIO), suggesting a deficiency in cytoplasmic organic anion-binding protein. Measurements of the conjugating enzymes are desirable. Conceivably, a decreased conjugating capacity could influence the uptake, as is suggested from studies on patients with Gilbert s syndrome (B14, B15,B18). 

Bilirubin is displaced from its binding protein by sulphonamides, vitamin K, X-ray contrast media or indomethacin in the neonate this may cause a significant risk of kernicterus, for its capacity to metabolise bilirubin is immature. 

S12, S42), but it is now known that albumin is not the governing factor, nor is it the most important factor to be considered in the accumulation of abnormal collections of body fluids (H8). Furthermore, it is apparent from the existence of relatively healthy adults with no albumin in the blood (B5, Bll) that the protein is not essential for life. Notwithstanding these lately appreciated facts, an important if not unique physiological role of albumin is its capacity to act as a vehicular substance for a wide variety of ions with different structures. In addition to its ability to bind and transport fatty acids, certain hormones, steroids, peptides, and amino acids, it can hold and effectively isolate the ions of many different substances that may be undesirable or dangerous to cellular life, e.g., phenols, phenolic acids, poisonous metals, bilirubin. 

CMPF accumulates in uremia and is poorly removed by hemodialysis, apparently because of its strong affinity for serum albumin (M4). It impairs albumin binding of several drugs, e.g., phenytoin and salicylate, and many endogenous substances such as tryptophan and bilirubin (M2), and in that capacity has been proposed as a potential uremic toxin. It is unique in being one of the few albumin-associated fluorescent substances known to be increased in uremic patients (Ml). 

By expressing drug concentrations as that required to displace 50 per cent of the baseline bound urate, the relative urate displacing capacities can be compared (Table IV). Aspirin is again the most potent urate displacer, probenecid the least. We have subsequently demonstrated a variety of agents which inhibit the binding of urate to plasma proteins and to date all such agents have been uricosuric. Other substances, such as bilirubin, have also been shown to interfere with the urate-plasma protein interaction. 

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