Molecular differences between rat-liver and rat-kidney biliverdin reductase. Implications for their in vivo regulation

Identification of specific protein markers in microdissected hepatocellular carcinoma

At present, the molecular mechanisms of hepatocellular carcinogenesis are not well-understood, and hepatocellular carcinoma (HCC) stays one of the most frequent and high-risk metastatic visceral neoplasms worldwide. For the identification of tumor-relevant proteins, we analyzed microdissected cells from nontumorous liver tissue (n = 28) and tissue derived from hepatic tumor center (n = 25), as well as tumor margin (n = 23). We unequivocally identified 53 proteins from hepatic tumor tissues by peptide fingerprint mapping and SELDI mass spectrometry that were separated using two-dimensional gel electrophoresis. Among a number of signals that were detected as significantly different in the protein profiling analysis, we identified for the first time ferritin light subunit (FLS) and adenylate kinase 3 alpha-like 1 (AK3), showing decreased expressions in hepatic tumor, as well as biliverdin reductase B (BVRB) that was upregulated in HCC. The use of ProteinChip technology in combination with tissue microdissection gives insight of the complex changes occurring at the protein level in hepatocellular cancer associated with tumor development and progression and resulted in three new potential diagnostically useful markers.

Cloning and characterization of the cDNA encoding human biliverdin-IX alpha reductase

Biliverdin reductase is classified into two isoforms in substrate specificity; biliverdin-IX alpha reductase and biliverdin-IX beta reductase with a molecular mass of 22 kDa and 34-42 kDa, respectively. We have cloned the cDNA encoding human biliverdin-IX alpha reductase from MOLT4 cDNA library. The cDNA of 1146 bp in nucleotide length contained an entire reading frame coding 296 amino acid residues. The NADH/NADPH binding consensus sequence was found in the amino-terminal region. Comparison between human and rat biliverdin-IX alpha reductases showed 82.8% identity in amino acid sequences and 80.3% identity in the coding nucleotides. The amino acid sequence of human biliverdin-IX alpha reductase showed no significant homology to that of human biliverdin-IX beta reductase. Northern blot analysis of poly(A) RNA from eight different human tissues revealed that the reductase mRNA was abundant in the brain, lung and pancreas but not in the liver. The distribution pattern of biliverdin-IX alpha message was different from that of heme oxygenase activity which is known to be high in the liver and to be low in the heart and lung.

Human biliverdin IXalpha reductase is a zinc-metalloprotein. Characterization of purified and Escherichia coli expressed enzymes

Biliverdin IXalpha reductase (BVR) catalyzes the conversion of the heme b degradation product, biliverdin, to bilirubin. BVR is unique among enzymes characterized to date in that it has dual pH/cofactor (NADH, NADPH) specificity. A cDNA clone encoding human BVR was isolated from a gamma library using a probe generated via reverse transcription and the polymerase chain reaction from human placental RNA. This approach was taken because the more direct approach of using the previously isolated rat BVR cDNA as the hybridization probe did not succeed. The human cDNA was cloned and sequenced; it was shown to have an open reading frame encoding a 296-amino-acid protein in which could be identified four peptides previously identified by micro-sequencing purified protein. The cDNA hybridized with a single message of approximately 1.2 kb in human kidney poly(A)-rich RNA, and appeared, by Southern blot analysis, to be the product of a single-copy gene. Sequence analysis indicated that the human reductase shows approximately 83% identity, at both the nucleotide and amino acid levels, with rat BVR. In some regions including the carboxyl terminus, protein sequence identity drops to 45%. Also noteworthy is the presence of two additional cysteine residues in the encoded human reductase (five compared to three for rat). The protein produced by an expression plasmid in which the insert was cloned in frame with lacZ sequences was characterized, and demonstrated dual pH and cofactor dependence. However, as suggested by kinetic analysis, the human enzyme may also use NADH as cofactor, as opposed to the rat reductase, which most likely utilizes only NADPH under physiological conditions. Western blot analysis and isoelectric focusing demonstrate that, although migrating as a single band on SDS/PAGE, the expressed protein, like that purified from tissue, consists of several isoelectric charge variants. Atomic absorption spectroscopy indicates that the protein purified from human liver contains Zn at an approximately 1:1 molar ratio. That human BVR is a Zn metalloprotein was further substantiated by 65Zn exchange analysis of both the purified and the fusion protein expressed in Escherichia coli. Exogenous Zn also inhibits NADPH-dependent, but not NADH-dependent, activity. Hence, the NADH and NADPH binding regions are differentiated by their ability to interact with Zn; Fe-hematoporphyrin, however, inhibited both NADH- and NADPH-dependent activity.

An analysis of the topography of molecular forms 1 and 3 of rat liver biliverdin reductase using polyclonal antibodies

Rat liver biliverdin reductase exists in two molecular forms. The major one (molecular form 1) is transformed, under conditions of oxidative stress into another molecular form (molecular form 3) which is an S-S bridged dimer of form 1. The chemical modifications of the thiol, arginine and lysine residues of molecular form 1 which resulted in an inhibition of its catalytic activity did not affect the activity of molecular form 3. Rabbit polyclonal antibodies raised against form 1 did not recognize form 3. This lack of recognition persisted even when the dimer (form 3) was denatured with SDS or urea under non-reductive conditions. Reduction of form 3 with reduced thioredoxin gave the monomeric form 1, which was fully recognized by the antibodies. The latter recognized the biliverdin reductases from rat spleen and kidney to the same extent as they did with form 1. Molecular form 1 was completely inhibited by the addition of the antibodies. This inhibition was prevented by preincubation of the enzyme with either the substrate (biliverdin) or the cosubstrate (NADPH). Preincubation with the latter or with NADP+ (but not with bilirubin) strongly impaired the recognition of form 1 by the antibodies. Modification of the lysine or arginine residues of form 1 which were involved in substrate binding, impaired the interaction of the enzyme with the antibodies. The antisera blocked the enzymatic conversion of form 1 to form 3, but alkylation of the thiol residue involved in this dimerization had no effect on the interaction of form 1 with the antibodies. The lack of recognition of form 3 by the antibodies suggest that the antigenic site of the former becomes buried upon dimerization.

The enzymatic and chemical reduction of extended biliverdins

The substrate specificity of rat liver biliverdin reductase was probed using helical and extended biliverdins. The former were the ZZZ-all-syn biliverdins IX alpha and IX gamma, and the latter were the 5Z-syn, 10Z-syn, 15Z-anti; 5Z-anti, 10E-anti, 15E-anti biliverdins. It was found that the reduction rates of the biliverdins increased with the progressive stretching of their conformations. The most extended biliverdin was reduced at a higher rate than biliverdin IX alpha. The chemical reduction rates to bilirubins followed a similar pattern. Nucleophilic addition of 2-mercaptoethanol to the C10 methine was also favored in the extended biliverdins.

Identification of the amino acid residues essential for the activity and the interconversion of the molecular forms of biliverdin reductase

Biliverdin reductase (molecular form 1, EC 1.3.1.24, bilirubin:NAD(P)+ oxidoreductase) carries three thiol residues. Only one of them could be alkylated when a ratio N-ethylmaleimide (NEM)/mol enzyme's SH = 90 was used. The alkylation of this thiol group inhibited the conversion of molecular form 1 to its dimer, molecular form 3; however, it did not inhibit the enzymatic activity. At a ratio of NEM/enzyme's SH = 300, two thiol residues were alkylated and the activity of the enzyme was totally inhibited. The third thiol group could not be alkylated either by NEM or by iodoacetamide. Biliverdin as well as the co-substrate NADPH protected the thiol residue essential for the enzymatic activity from alkylation. Spectroscopic evidence was obtained that this thiol group binds covalently to the C-10 of biliverdin to form a rubinoid adduct. The presence of a lysine residue, which is also essential for the enzymatic activity, could be inferred from the fact that by reduction of the Schiff base formed by the enzyme with pyridoxal phosphate the catalytic activity was irreversibly abolished. The location of a lysine residue in the vicinity of the thiol group involved in the catalytic activity was evident when the enzyme was treated with o-phthalaldehyde. The inactivation of the enzymatic activity was coincident with the formation of the fluorescent isoindole derivative which originates when the thiol and epsilon-NH2 groups are located about 3 A apart. The presence of a positively charged ammonium ion in the vicinity of the NADPH binding site was inferred from the shifts in the UVmax of NADPH from 340 nm to 327 nm and of 3-acetyl NADPH from 360 nm to 348 nm when the pyridine nucleotides bind to the reductase. The involvement of arginine residues in the enzymatic activity was established by inhibition of the latter after reaction with butanedione. This inhibition was totally protected by NADPH but not by biliverdin. The similarity of the structural features of biliverdin reductase with those of several dehydrogenases is discussed.

The in vivo and in vitro oxidation of molecular form 1 of biliverdin reductase to molecular form 3 by diamide

Administration of phenylhydrazine to rats converted molecular form 1 of the liver biliverdin reductase into its disulfide bridged dimer (molecular form 3). This oxidative dimerization was shown not to be mediated by the NAD(+)-dependent dehydrogenase [(1984) Biochem. Biophys. Res. Commun. 121, 249-254]. Administration of diamide produced the same conversion. Although hepatic levels of GSH also decreased, no mixed disulfides of the reductase and GSH could be detected. Administration of the antioxidants allopurinol and alpha-tocopherol together with the diamide did not affect this conversion of molecular forms produced by the latter. The diamide also oxidized molecular form 1 of biliverdin reductase in vitro and molecular form 3 was formed. The chemical oxidation took place at a high rate and was partially inhibited by GSH but not by cysteine.