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Catabolism of Tetrapyrroles As the Final Product of Heme Catabolism (Cf Scheme 1)

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Catabolism of Tetrapyrroles As the Final Product of Heme Catabolism (Cf Scheme 1) CHEMIE IN FREIBURG/CHIMIE A FRIBOURG 352 CHIMIA 48 (199~) Nr. 9 (Scl'lcmhcr) ns itu Chimia 48 (/994) 352-36/ heme (1), at the a-methene bridge (C(5)) €> Neue Sclnveizerische Chemische Gesellschaft producing CO and an unstable Felli com- /SSN 0009-4293 plex. The latter loses the metal ion to yield the green pigment protobiliverdin IXa (usually abbreviated to biliverdin (2)), which is excreted by birds and amphibia, Catabolism of Tetrapyrroles as the final product of heme catabolism (cf Scheme 1). The iron is recovered in the protein called ferritin and can be reutilized Albert Gossauer* for the biosynthesis of new heme mole- cules. As biliverdin (2) has been recog- nized to be a precursor in the biosynthesis of phycobilins [9], a similar pathway is Abstract. The enzymatic degradation of naturally occurring tetrapyrrolic pigments probably followed for the biosynthesis of (heme, chlorophylls, and vitamin B 12) is shortly reviewed. this class oflight-harvesting chromophores 1. Introduction pounds known so far are synthesized, have Scheme I. Catabolism (!{ Heme ill Mammals been already elucidated, it may be antici- In contrast to the enormous amount of pated that the study of catabolic processes work accomplished by chemists in the will attract the interest of more chemists elucidation of biosynthetic pathways of and biochemists in the near future. secondary metabolites (terpenes, steroids, alkaloids, among others), only a few at- tempts have been made until now to un- 2. Heme Catabolism derstand the mechanisms oftheirdegrada- tion in living organisms. A possible rea- It has been known for over half a cen- son for this fact is the irrational association tury that heme, the oxygen-carrier mole- of degradation (catabolism: greek Kara= cule associated with the blood pigment down) with decay and, thus, with unattrac- hemoglobin, is converted in animal cells tive dirty colors and unpleasant odors. The to bile pigments, which are excreted with other reason is probably the meaning that the faeces and urine [I]. Owing to its degradation processes occur outside the significance in human medicine, the ca- cell rather by uncontrolled chemical reac- tabolism of the heme released upon the tions which are achieved by environmen- degradation of hemoglobin and related tal factors such as oxygen, water, temper- chromoproteins, like myoglobin and cyto- ature, etc., than by catabolic (i.e., enzyme- chromes, has been intensively studied catalyzed) processes. Thus, with excep- during the past decades. Particularly, in tion of proteins, fats, and sugars, which are recent years the application of modem subjected to a continuous degradation and spectroscopic, chromatographic, and iso- rebuilding in the living organism, and of topic techniques as well as the develop- nucleic acids, the well-documented catab- ment of methodologies which made the olism of which is treated in all text books most important bile pigments like biliver- on biochemistry, almost nothing is known din [2], bilirubin [3], phycocyanobilin (Proto)biliverdin IXa (2) concerning the fate of most of the compo- [4][5], phycoerythrobilin [6][7], and phy- nents ofliving organisms which are liber- tochromobilin [8] accessible by chemical ated from the organelles in the course of synthesis have enabled to elucidate a morphological differentiation of their cells. number of central aspects of the problem Actually, as the main pathways ofbiogen- but made others, of course, still more esis, by which virtually all natural com- puzzling. Ordinarily, heme degradation occurs in the phagocytic cells of the bone mar- row, spleen, and liver but can probably ·Correspondence: Prof. Dr. A. Gossauer Institut fUr Organise he Chemie occur in macrophages in other tissues as UniversiUit Freiburg i. Ue. well. The catabolic process is initiated by Ch. du Musee 9 the oxidative scission of the protoporphy- (Proto)bilirubin IXa (3) CH-1700 Fribourg rin IX macrocycle, the chromophore of CHEMIE IN FREIBURG/CHIMIE A FRIBOURG 353 CHI~lIA 48 t 144-1.)Nr. l) (Sl'Ph..'lllht'r) which are associated with photosynthesis Scheme 2. Hypothetical Mechanismforthe Catabolism of Heme to Bilh'erdinlXa in vivo. Side chains in cyanobacteria and in two groups of and apoprotein have been omitted for clarity. eukaryotic algae, namely the red algae and the cryptomonads. H0PH Moreover, the occurrence of bile pig- heme -°0 ments in arthropoda (insects, crabs, oxygenas: n:S)0 shrimps, and barnacles), mollusca (squids, L'" ~:(I~;-~ clams, snails, etc.), as well as in egg shells, 4 / " fins, and scales has been recognized for a long time. Thus, e.g., biliverdin (2) is ° responsible for the green color of the pray- ing mantis (Mantis religiosa L.); one of its NADPH regioisomers, pterobilin (protobiliverdin ..• IX il, has been isolated from the wings of certain butterflies, another regioisomer (protobiliverdin IXO) from the ovaries of 5 the marine snail Turbo cornutus [10]. At heme Ioxygenase least for insects of two different taxonom- ° ic categories (Mantis religiosa L., which H,0",0, feeds on other insects, and Locusta migra- 00) H toria L., a herbivore), de novo biosynthe- sis of endogenous biliverdin has been dem- • onstrated [1 I]. As insects do not contain "F~llI) ~>4~~/" heme in their hemolymph, either a biosyn- thetic pathway peculiar to them or degra- dation of cytochromes may be taken into /O,(!)",H H (0 heme consideration to explain the presence of oxygenase biliverdin (2) in their tissues. In humans •• and mammals, enzymatic reduction of ~(~;RN" / ~ /Fe~l) ~ biliverdin, which is catalyzed by the NADPH-dependent enzyme biliverdin reductase, yields yellow bilirubin (strictly speaking protobilirubin TXa (3». Actual- H I ly, catabolism of heme is the only known (0 source of bilirubin (3), of which 300-500 ° mg are produced daily in adult human •• ,,1/ . H2O beings. Once formed, 3 is esterified with ~ ~ Fe(lIl) ~ ~ /"-... various sugars (mainly glucuronic acid) in 8 the liver, and these water-soluble esters (so-called bilirubin conjugates) are ex- creted via the bile duct into the gut where they undergo further reductive transfor- mations to urobilinoids by the intestinal Scheme 3. Possible Pathway of the Mode of Action of Heme Hydrogenase (side-view ofthe porphyrin flora, before their final excretion in the iron-complex chromophore represented by a bar) faeces. On the basis of the experimental evi- dence available at present, Scheme 2 is I 1111 proposed as an outline of the chemical Felli'0-0' Fe steps involved in the conversion of heme I I to biliverdin in vivo (cf [9]). The mode of Heme Heme action of microsomal heme oxygenase oxygenase may be analogous to the commonly ac- cepted mechanism of the autoxidation of ferrous porphyrins in vitro, as represented in Scheme 3 ((-1 [12]). Thus, the suggested estingly, heme oxygenase is inactive reducing agent (usually ascorbic acid). pathway explains the requirement for against protoporphyrin IX, suggesting that This reaction, named 'coupled oxidation' molecular oxygen and provides an expla- the central Fe-atom of heme is essential because it requires both oxygen and a nation for the need for NADPH and a for the activity of the enzyme. reductant, was thought in the past to be a functioning microsomal electron-transport It is known that pyridino-ferrous-por- suitable model for the enzymatic break- system. Moreover, it is consistent with the phyrin complexes (so-called pyridine he- down of heme, in which basic amino-acid finding that the microsomal heme oxyge- mochromes (4), L = pyridine) are readily residues of the apoprotein replace pyrid- nase is inhibited by CO as well as by azide oxidized and cleaved to give bile pigment ine as stabilizing ligands for ferrous iron and cyanide ions, which may interact with precursors by mild treatment with oxygen and NADPH acts as the reducing agent. the ferrous heme-protein complex. Inter- in the presence of an excess of a suitable However, after the origin of the a-atoms CHEMIE IN FREIBURG/CHIMIE A FRIBOURG 354 CHIMIA 18 (1994) Nr. 9 \Scplcl11ocr) present in the two lactam groups of the atmosphere inappreciable amounts. There- biliary excretion, and5) transport and elim- biliverdin molecule has been soundly es- fore, it is particularly interesting, in this ination in the intestine. Bilirubin (3) re- tablished by a series of outstanding exper- connection that bile pigments (so-called leased into the plasma is firmly bound to iments using oxygen isotopes by Brown bactobilins) are formed from uroporphy- plasma albumin, which serves as its carri- and coworkers [13-15], the mechanism of rins I and III by cell extracts of Clostrid- er protein. The mechanism by which un- the enzymatic reaction appears to be more ium tetanomorphum under anaerobic con- conjugated bilirubin is removed from al- complex. According to these investiga- ditions [18][19]. bumin and transmitted into the liver cells tions, the cleavage of the protoporphyrin Tn summary, the mechanism of ring is unknown. Indeed, it is not even known macrocycle occurs by a 'two-molecule cleavage of heme is highly unusual. The with certainty, whether the hepatic uptake mechanism', i.e., the terminallactam 0- coupled oxidation reaction is unique in of3 is an active or a passive process. In the atoms in biliverdin (2) are derived from porphyrin chemistry in that it permits this liver, 3 undergoes ester formation with a two different O2 molecules. Thus, the for- particularly stable macrocycle to be easily variety of carbohydrate moieties and pos- mation of2 by a hydrolytic step involving opened under extremely mild conditions. sibly with other conjugating groups that the insertion of an a-atom from the medi- Furthermore, decarbonylation at room tem- render the pigment water-soluble. The so- um does not occur in the heme-oxygenase perature in the dark is a rare reaction in called conjugation of the bile pigment is system. This reaction pathway (7 ~ 8) has organic chemistry, specially in the con- catalyzed by membrane-bound enzymes been observed, however, in the coupled densed phase.
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