Biliverdin reductase

Hyjjerbilirubinaemia is an abnormality observed mainly in neonates in whom the liver is insufficiently developed to be able to detoxify the bile pigment bilirubin. This situation is known as neonatal jaundice and can sometimes become a serious disease causing neurotoxic symptoms. Bilirubin is produced by the degradation of heme [the Fe(II) complex of protoporphyrin IX] by heme oxygenase to give biliverdin, which is reduced by biliverdin reductase to... 

In birds and amphibia, the green biliverdin IX is excreted in mammals, a soluble enzyme called biliverdin reductase reduces the methenyl bridge between pyrrole III and pyrrole IV to a methylene group to produce bilirubin, a yellow pigment (Figure 32-12). 

BIFUNCTIONAL CATALYSIS BIFUNCTIONAL ENZYME BIFURCATION THEORY BILIVERDIN REDUCTASE BIMOLECULAR... 

Biliverdin is converted to bilirubin in the second step, catalyzed by biliverdin reductase. You can monitor this reaction colorimetrically in a familiar in situ experiment. When you are bruised, the black and/or purple color results from hemoglobin released from damaged erythrocytes. Over time, the color changes to the green of biliverdin, and then to the yellow of bilirubin. Biliru-... 

Baranano DE, Rao M, Ferris CD, Snyder SH. 2002. Biliverdin reductase A major physiologic cytoprotectant. Proc Natl Acad Sci 99 16093-16098. 

Noncovalent functional strategies to modify the outer surface of CNTs in order to preserve the sp2 network of carbon nanotubes are attractive and represent an effective alternative for sidewall functionalization. Some molecules, including small gas molecules [195], anthracene derivatives [196-198] and polymer molecules [118, 199], have been found liable to absorb to or wrap around CNTs. Nanotubes can be transferred to the aqueous phase through noncovalent functionalization of surface-active molecules such as SDS or benzylalkonium chloride for purification [200-202]. With the surfactant Triton X-100 [203], the surfaces of the CNTs were changed from hydrophobic to hydrophilic, thus allowing the hydrophilic surface of the conjugate to interact with the hydrophilic surface of biliverdin reductase to create a water-soluble complex of the immobilized enzyme [203]. 

Figure 22-1. Production of bilirubin (BR). The degradation of I e l 3-hcme by molecular oxygen and NAD PH, catalyzed by microsomal heme oxygenase, produces biUverdin, CO, and Fe+2. Subsequent reduction of biliverdin by NADPH, catalyzed by biliverdin reductase, produces bilirubin. Bilirubin that is produced in phagocytes from degradation of senescent erythrocytes is transported to liver for conjugation with glucuronic acid, catalyzed by bilirubin-UDP-glucuronyltransferase. In some cells, the bilirubin is used as an antioxidant, where it recycles through the biliverdin reductase reaction.

The Unear tetrapyrrole biUverdin IXa that is a product of the heme oxygenase reaction is the substrate for biliverdin reductase, which catalyzes the NADPH-dependent reduction of biUverdin IXa at the y-methene bridge to form bilirubin IXa (Fig. 22-1). Many of the pathological problems associated with excess production or diminished eUmination of biUrubin are... 

The widespread expression of biliverdin reductase, which parallels the widespread expression of heme oxygenase, suggests that bilirubin may play a special role in particular tissues. Studies suggested that bilirubin is an excellent lipid-soluble antioxidant. Bilirubin reacts with reactive oxygen species (ROS) that oxidize bilirubin back to biliverdin. The biliverdin then is reduced again to bilirubin. This repeated cycling results in the net use of NADPH to protect cells from ROS (Fig. 22-1). This system may be especially important in neurons. 

During the next weeks, this assay should be modified further especially concerning the ratio of hemin concentration and used volume of mierosomal fraction. Because of the pH dependency of biliverdine reductase (XXX) the pH should be checked at every step. The optimal pH for using NADPH as cofactor is 8.7 (Maines et al. 1988). 

The 1 mL reaction mixture contained heme oxygenase, 5 mM deferroxa-mine, 25 piM hemin, 15 fiM bovine serum albumin, 1 mM NADPH, 0.1 M potassium phosphate (pH 7.4), and 0.05 mL of 105,000g supernate from 20% (w/v) perfused rat liver homogenate as a source of biliverdin reductase. To stop the reaction, 0.1 mL of the reaction mixture was added to 0.1 mL of ethanol-DMSO (95 5, v/v). After 2 minutes incubation on ice, the mixture was centrifuged and 0.1 mL of the supernate was injected on HPLC. The rate of bilirubin formation was linear up to 0.63 mg protein/0.237 mL assay, and up to 9 minutes incubation at 37°C. 

The catabolism of haemoglobin yields haem, which is subsequently converted to bilirubin in a two-step process that takes place in the hepatocyte. First, the microsomal enzyme haem oxygenase cleaves the porphyrin ring of haem, generating biliverdin in an energy-utilising reaction. Following this, biliverdin is converted to bilirubin by the cytosolic enzyme biliverdin reductase. As the liver is the active site for biosynthesis of porphyrin and haem, deficiencies in some enzymes of the porphyrin pathway may lead to insufficient haem production and an increase in porphyrin levels, which causes acute porphyria attacks. 

The oxidation of haem, a complex consisting of iron and protoporphyrin which is derived from haemoglobin, is effected by haem oxygenase to produce biliverdin IXa. This is converted to bilirubin IXa by biliverdin reductase. The reaction speed of this process is governed by the haem oxygenase. This enzyme complex contains the inducible cytochrome P 450, which accelerates bilirubin production when the haemoglobin level is elevated. A small proportion of bilirubin (20-30%) is produced from the degradation of other metalloporphyrins. (s. fig. 3.1)... 

Heme oxygenase, which catalyzes the conversion of free heme groups to biliverdin and CO, functions as part of a microsomal electron transport system similar to that of cytochrome 1 450-(FP = NADPH-cytochrome P450 reductase.) Heme oxygenase requires 3 02 and 5 NADPH. Biliverdin reductase can use NADPH or NADH as a reductant. 

The porphyrin heme is degraded to form the excretory product bilirubin in a biotransformation process that involves the enzymes heme oxygenase and biliverdin reductase and UDP-glucuronosyltransferase. After undergoing a conjugation reaction, bilirubin is excreted as a component of bile. 

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