The enzymatic formation of bilirubin

Comparative aspects of bile pigment formation and excretion

Breakdown of haem which is of key importance in most organisms, involves oxidative CO-evolving cleavage of the macrocyclic ring with formation of biliverdin-IX. In two major pathways established so far formation of biliverdin-IX alpha is followed by (a) biliary secretion or (b) reduction to bilirubin-IX alpha, formation of more hydrophilic derivatives (usually glycosidic conjugates) and biliary secretion. The scattered comparative information available indicates marked species variation with regard to the methin-bridge carbon atom removed from haem and the metabolic site of cleavage, the nature of bilirubin conjugates and the developmental sequence of maturation of enzyme activities and transport proteins involved in the chain of events leading from breakdown of haem to the excretion of the final end products.

Bile-pigment formation from different leghaemoglobins. Methine-bridge specificity of coupled oxidation

The coupled oxidation of leghaemoglobins with O(2) and ascorbate yielded oxyleghaemoglobin in the first reaction step, and the second step was the degradation of haem characterized by an A(675) increase. Leghaemoglobins were degraded to biliverdin isomers specifically, depending on the structure of the protein. The main leghaemoglobin components of Glycine (soya bean) and Phaseolus (kidney bean) were degraded to biliverdin mixtures containing about 50% of the beta-form, about 30% of the alpha-form and about 20% of the delta-isomer, whereas the leghaemoglobin I components of Vicia (broad bean) and Pisum (pea) were degraded almost exclusively to the beta-isomer, with traces of the alpha-isomer. The amino acid sequences of Glycine and Phaseolus leghaemoglobins resemble each other, as do those of Vicia and Pisum. The site specificity of bile-pigment formation from leghaemoglobins can be tentatively explained by specific differences in the amino acid sequences at those regions of the polypeptide chain that are in the vicinity of the appropriate methine bridges. The ligand-binding site in different leghaemoglobins may be outlined on the basis of the present results, supposing that the haem is degraded when a reduction product of haem-bound O(2) reacts with a methine bridge of the haem, and that the bridge specificity is regulated by hindering amino acid residues that determine the location of the bound O(2). The residue phenylalanine-CD1 appears to be further away from the haem plane or in a markedly more flexible position in leghaemoglobins than in mammalian globins. The haem-bound oxygen atom B, in Fe-O(A)-O(B), seems to be free to rotate in all directions except that of the gamma-bridge in Glycine and Phaseolus leghaemoglobins, but its position in Vicia and Pisum leghaemoglobin I might be restricted to the direction of the beta-methine bridge.

Oxidation of glycine by Phaseolus leghaemoglobin with associated catabolic reactions at the haem

Leghaemoglobin from the root nodules of kidney bean (Phaseolus vulgaris) reacts in alkaline glycine solutions as a glycine oxidase in a reaction that may also be regarded as a coupled oxidation. Leghaemoglobin is reduced to the ferrous form by glycinate, the oxygen complex is formed, and finally the haem is attacked to yield a green reaction product. Glycine is simultaneously oxidized to glyoxylate, and hydrogen peroxide is generated. The initial velocity of the formation of the green product is proportional to the concentrations of leghaemoglobin and glycine, and the optimum pH for the reaction is 10.2. The green product is not formed if carbon monoxide, azide of imidazole is bound to the haem, whereas oxidation of glycine to glyoxylate is not inhibited by azide and not essentially by carbon monoxide. Haem breakdown is activated by digestion of leghaemoglobin by carboxypeptidase, and partly inhibited by catalase and superoxide dismutase.

The mechanism of haem degradation in vitro. Kinetic evidence for the formation of a haem-oxygen complex

The rate of haem degradation in aqueous pyridine shows a saturation dependence on O2 concentration. CO competitively inhibits the reaction. This is evidence for formation of an iron-O2 complex and is consistent with an intramolecular pathway for haem degradation.

Oxidative degradation of haemoglobin by nitrosobenzene in the erythrocyte

Substitutions on the benzene ring of nitrosobenzene did not have the same effect on oxidative haemolysis as substitutions on phenylhydrazine. We previously found that the haemolytic effect of arylhydrazines paralleled their oxidative conversion into ligands of ferrihaemoglobin. In contrast, although most substituted nitrosobenzenes that are ligands of ferrohaemoglobin caused haemolysis and most that are not ligands failed to cause nitrosoarenes appeared to be related more closely to the ease of their reduction to arylhydroxylamines than to their properties as ligands. We propose a mechanism of oxidative degradation whereby the cyclic formation of phenylhydroxylamine from nitrosobenzene within an erythrocyte leads to the accumulation of H2O2, which then reacts with ferrohaemoglobin to initiate the oxidative cleavage of haem. The posulated active intermediate in this reaction is the same as that previously proposed in the oxidative degradation of haemoglobin by phenylhydrzine and in the coupled oxidation of ascorbic acid and haemoglobin.

Haem degradation in abnormal haemoglobins

The coupled oxidation of certain abnormal haemoglobins leads to different bile-pigment isomer distributions from that of normal haemoglobin. The isomer pattern may be correlated with the structure of the abnormal haemoglobin in the neighbourhood of the haem pocket. This is support for haem degradation by an intramolecular reaction.

The mechanism of haem catabolism. A study of haem breakdown in spleen microsomal fraction and in a model system by 18O labelling and metal substitution

The mechanism of bile-pigment formation from haem breakdown was studied by using 18O labelling of the molecular oxygen required for macrocyclic ring cleavage. For haem degradation by the spleen microsomal haem oxygenase system, mass spectrometry of the product bilirubin revealed that cleavage occurred by the Two-Molecule Mechanism, i.e. the terminal lactam oxygen atoms in bilirubin were derived from two different oxygen molecules. Similarly, degradation of myoglobin by coupled oxidation with ascorbate and oxygen proceeded via the Two-Molecule Mechanism. Cobalt and manganese complexes of protoporphyrin IX were not degraded by either the haem oxygenase system or the coupled oxidation system. This result suggests that the iron atom possesses unique properties in facilitating porphyrin breakdown.

Twenty-five years of progress in bilirubin metabolism (1952-77)

This review deals with the development of our understanding of the chemistry of bilirubin and its glucuronide derivatives during the years 1952-1977. It examines the relation between haem metabolism and bilirubin formation and our present knowledge of hepatic transport of bilirubin. The heterogeneity of familial hyperbilirubinaemia is discussed.

The mechanism of haem catabolism. Bilirubin formation in living rats by [18O]oxygen labelling

1. The pathway of haem breakdown in living rats was studied by using 18O in the oxygen that the animals consumed. By cannulation of the common bile duct and collection of bile, labelled bilirubin was isolated and its mass spectrum determined. One set of results was obtained for a rat to which haemoglobin had been intravenously administered and another set obtained for a rat that was not given exogenous haem. Isomerization of bilirubin IXalpha to the XIIIalpha and IIIalpha isomers did not occur to any significant extent. The 18O-labelling pattern obtained in the bilirubin was consistent with a Two-Molecule Mechanism, whereby the terminal lactam oxygen atoms of bilirubin are derived from different oxygen molecules. The consequences of this mechanism are discussed in terms of the possible intermediates of the catabolic pathway. 2. 18O-labelled bilirubin appeared in the bile in less than 10 min after exposure of the animals to labelled oxygen. This result suggests that all of the chemical transformations involving production of biliverdin, reduction to bilirubin and conjugation of the bilirubin are fast processes. 3. The quantitative recovery of label obtained in the experiments suggests that there is little or no exchange of newly synthesized bilirubin with existing bilirubin pools in the animal.

A new method for determination of microsomal haem oxygenase (EC 1.14.99.3) based on quantitation of carbon monoxide formation

Microsomal haem oxygenase (MHO) catalyses the main pathway for haem degradation. Hitherto bilirubin formation rate (BFR) has been used for determination of MHO activity. In the present study a method is described where MHO activity is assessed from the rate of carbon monoxide formation (VCO) in tissue homogenates with methaemalbumin as substrate. Formed CO is bound to haemoglobin present in the tissue homogenate. CO is measured by gas chromatography after reduction to methane. CO amounts of 0.01 nmol could be measured. This corresponded to an MHO activity of 0.05 pkat when using standard incubation mixture and 12 min incubation time. The correlation with MHO activity determined with the BFR technique was good (r = 0.94; n = 41); MHOVCO = 0.94-MHOBFR-0.08). Advantages with the VCO method are a ten-fold increase in sensitivity compared to the BFR method and independence of biliverdin reductase.

The oxygen sensing characteristics of microsomal enzymes

The microsomal electron transport complex, in particular the segment associated with cytochrome P-450 function, is both qualitatively and quantitatively an important contributor to the cellular respiration of tissues such as liver. Although our knowledge is still limited, it is apparent that oxygen plays a pivitol role in dictating the mode of substrate hydroxylation or the generation of hydrogen peroxide. As illustrated in Figure 7, current evidence suggests that hydrogen peroxide is formed by the dismutation of the superoxide anion resulting from the dissociation of oxycytochrome P-450. Of interest are recent studies demonstrating the ability of hydrogen peroxide to initiate a cytochrome P-450 dependent peroxidatic reaction competent for supporting substrate hydroxylation. The fact that the function of cytochrome P-450 is sensitive to changes in oxygen tension establishes its role as an "oxygen sensor" for cellular metabolism.

Stereospecific haem cleavage. A model for the formation of bile-pigment isomers in vivo and in vitro

A new approach is suggested for an explanation of sterospecific haem degradation to biliverdin and bilirubin. A model is proposed in which an oxygen molecule, bound to the haem iron atom, attacks a methene-bridge carbon atom in an intramolecular reaction. Specificity of macrocyclic ring cleavage is explained on the basis of the different accessibilities of the bound oxygen molecule to the four methene bridges. The consequences of these ideas are assessed in relation to coupled oxidation in model systems and to haem catabolism.