Synthesis of isoornithines and methylputrescines. An evaluation of their inhibitory effects on ornithine decarboxylase.

2-(Aminomethyl)-4-aminobutyric acid (isoornithine), 3-methylisoornithine, and 2,3-dimethylisoornithine were not decarboxylated by liver ornithine decarboxylase (ODC, EC 4.1.1.17) of thioacetamide-treated rats but were good competitive inhibitors of the enzyme (Ki ranged from 0.72 to 1.79 mM). When assayed in vivo in the treated rats, the above mentioned isoornithines were also found to inhibit liver ODC when administered 1 h before sacrifice. When the methylputrescines formally derived from the decarboxylation of several isoornithines were assayed on rat liver ODC, it was found that only 2,3-dimethylputrescine decreased the enzymatic activity. When assayed in vivo, it was found to decrease ODC activity by 60%, when the latter was measured 1 h after administration. The effect was reverted 4 h after administration of the drug. Isoornithines were not taken up by H-35 hepatoma cells; hence they did not affect their ODC activity. 2,3-Dimethylputrescine however, was transported into the cells and significantly decreased its ODC activity.

Degradation of heme IX in rats pretreated with cobaltous chloride. A study in isolated perfused liver.

The effect of the administration of cobaltous chloride on the degradation of heme IX was investigated using perfusions of isolated rat livers. The presence of biliverdin IX beta (2%) in the bile fluid of cobaltous chloride treated rats support the hypothesis of the presence of a chemical oxidation of heme IX induced by the cobalt salt. In control rats the absence of biliverdin IX beta in the bile fluid was observed. When biliverdin IX beta was added to the perfusate it was excreted in the bile fluid as bilirubin IX beta without conjugation. An increase from approximately twofold to three-fold of the total bilirubins in the bile fluids of cobaltous chloride pretreated rats in relation with control rats was observed due to heme oxygenase enhanced activity. Bilirubin IX alpha diconjugates increased ca. 15% in the bile fluids of cobaltous chloride pretreated rats, after the addition of hemin IX. The increase could reflect the presence of a new molecular form of UDP-glucuronosyltransferase, which favours the formation of bilirubin IX alpha diglucuronide and therefore its preferential excretion into the bile. In the bile fluid of the rats pretreated with cobaltous chloride, other diconjugates of bilirubin IX alpha were also detected 120 min after the addition of hemin IX to the perfusate, consisting of glucose and glucuronate (5%) and xylose and glucuronate (5%).

Structure/activity relationships in porphobilinogen oxygenase and horseradish peroxidase. An analysis using synthetic hemins.

The apo-enzymes of porphobilinogen oxygenase and horseradish peroxidase were reconstituted with hemin IX, deuterohemin IX, 2,4-diacetyldeuterohemin IX, 2-vinyl-4-deuterohemin IX and hemin I. The apoproteins did not reconstitute with the dimethyl or diethyl esters of hemin IX. The native enzymes and the synthetic hemoproteins showed similar oxygenase activities toward porphobilinogen in the presence of dithionite and oxygen. They also showed peroxidase activity in the presence of H2O2, which was affected by the side-chain substitution pattern of the hemes. Oxygenase activities, however, were not affected by the heme structure. Iron chelators completely inhibited the oxygenase, but not the peroxidase activities. The EPR spectra of the native and synthetic porphobilinogen oxygenase showed that dithionite reduction produced a rapid disappearance of the high-spin heme-iron signal at g = 6.0. It reappeared 1 min later but the enzyme retained its catalytic activity. The changes in the EPR spectra could be correlated with the biphasic kinetics of the oxygenase reaction which was very fast during the first minute and then decreased to a half-value rate. The oxygenase reaction was inhibited by addition of superoxide dismutase during the fast rate phase, but not during the slower phase. These results could be explained by the formation of a superoxide anion during the first minute of the oxygenase reaction, after which a protein-stabilized radical (g = 2.0) is generated (very likely a tyrosyl radical). The latter then oxidizes the substrate porphobilinogen and facilitates its reaction with O2 to give oxopyrrolenines.

The interplay between basicity, conformation, and enzymatic reduction in biliverdins.

Biliverdins with extended conformations are reduced by biliverdin reductase (BvR) at higher rates than biliverdins with helical conformations. To find out the molecular basis for this important feature of BvR mechanism, helical and extended biliverdins were titrated for their acid-base equilibria in a protic solvent (methanol). It was found that the basicity of biliverdins increases with the stretching of the conformation. Biliverdin IX gamma (all-syn) has a pKa = 3.6; 5,10,15-syn,syn,anti-biliverdin has a pKa = 3.7; 5,10,15-syn,anti,syn-biliverdin has a pKa = 6.1; 5,10,15-syn,anti,anti-biliverdin has a pKa = 6.4; and 5,10,15-all-anti-biliverdin has a pKa = 7.9. The increase in basicity with progressive stretching of conformations closely parallels the increase in the reduction rates by BvR. A biliverdin constrained by a four carbon chain to a helical conformation and which is a very weak base (pKa = 0.4) is not reduced by BvR. Nucleophilic additions of 2-mercaptoethanol at the C10 in biliverdins closely parallel their basicities, as can be expected if the formation of a positive mesomeric species at C10 is linked to the basicity (i.e., the ease of protonation) of the N23 on the pyrrolenine ring.

Reconstitution of apomyoglobin with extended biliverdins.

An analysis of the reconstitution of biliverdins with extended conformations and horse heart apomyoglobin was carried out. Biliverdins with the 5Z-syn, 10Z-syn, 15Z-anti and 5Z-anti, 10Z-syn, 15Z-anti conformations, as well as biliverdins with the Z,Z,Z, all-syn conformation recombined with apomyoglobin. In every case the P enantiomers were bound in excess to the M enantiomers, with exception of the 5-syn, 10-syn, 15-anti biliverdin where the M enantiomer bound preferentially to the protein. Biliverdins with an anti conformation at the C-10 meso bridge did not recombine with the protein. It was concluded that the presence of a syn conformation at the C-10 methine conferred to the biliverdin the necessary helicity to fit into the apomyoglobin heme pocket. This regioselectivity is of importance in view of the well known analogy between the ligand domains of myoglobin and the C-phycocyanins.

Cadaverine, an Essential Diamine for the Normal Root Development of Germinating Soybean (Glycine max) Seeds.

When the polyamine content of soybean (Glycine max) seeds was examined during the early stages of germination, the major polyamine in the cotyledons was found to be spermidine, followed by spermine; while very low concentrations of cadaverine were found. In the embryonic axes, however, cadaverine was the main polyamine and its content markedly increased 24 hours after the start of germination. When the germination of the seeds was performed in the presence of 1 millimolar alpha-difluoromethylornithine (DFMO), a marked decrease in the cadaverine content was found, while the other polyamines were not affected. This decrease of the cadaverine content was already noticeable after the first hours of germination. In the presence of DFMO, a pronounced elongation in the roots of the seedlings and a marked decrease in the appearance of secondary roots as compared with controls, was observed. This abnormal rooting of the seedlings caused by DFMO was almost completely reverted by the addition of 1 millimolar cadaverine. The latter also increased the appearance of secondary roots in the seedlings. The decrease in the cadaverine content produced by DFMO could be traced to a strong inhibition of lysine decarboxylase. A temporal correlation between the increase in cadaverine content and the increase in lysine decarboxylase activity was found. Both reached a maximum at the second day of germination. The activity of diamine oxidase, the cadaverine degrading enzyme, started to increase at the third day and reached a maximum between the fourth and fifth day of germination. DFMO increased the activity of diamine oxidase by about 25%. Hence, the large decrease in cadaverine content produced by DFMO has to be attributed to the in vivo suppression of lysine decarboxylase activity. Ornithine decarboxylase activity was also suppressed by DFMO, but putrescine and spermidine contents were not affected, except in the meristematic tissues. The obtained results suggest an important role for cadaverine in the normal rooting process of soybean seedlings.

The dual oxygenase and peroxidase activities of porphobilinogen oxygenase and horseradish peroxidase: a study using the reaction with phenylhydrazine.

Porphobilinogen oxygenase and horseradish peroxidase show dual oxygenase and peroxidase activities. By treating porphobilinogen oxygenase with phenylhydrazine in the presence of H2O2 both activities were inhibited. When horseradish peroxidase was treated in the same manner only the peroxidase activity was lost while its oxygenase activity toward porphobilinogen remained unchanged. The phenylhydrazine treatment alkylated the prosthetic heme group of porphobilinogen oxygenase and N-phenylheme as well as N-phenylprotoporphyrin IX were isolated from the treated hemoprotein. In horseradish peroxidase the modified heme was mainly 8-hydroxymethylheme. The apoproteins of the alkylated enzymes were isolated and recombined with hemin IX. The oxygenase and peroxidase activities of porphobilinogen oxygenase were entirely recovered in the reconstituted enzyme, while the reconstituted horseradish peroxidase regained 75% of its peroxidase activity.

Heme oxygenase induction by CoCl2, Co-protoporphyrin IX, phenylhydrazine, and diamide: evidence for oxidative stress involvement.

The induction of heme oxygenase in rat liver by cobaltous chloride (CoCl2) and Co-protoporphyrin IX is entirely prevented by the administration of alpha-tocopherol and allopurinol. CoCl2 was converted in the liver into Co-protoporphyrin IX before it induced heme oxygenase activity. Actinomycin and cycloheximide affected to a similar degree the induction of heme oxygenase by both CoCl2 and Co-protoporphyrin IX. Administration of either CoCl2 or Co-protoporphyrin strongly decreased the intrahepatic GSH pool, a decrease which was completely prevented by the administration of either alpha-tocopherol or allopurinol. The latter compounds prevented heme oxygenase induction as well as the decrease in hepatic GSH when administered 2 h before, together with, or 2 h after CoCl2. However, when given 5 h after administration of CoCl2, alpha-tocopherol and allopurinol showed no preventive effect. Similar results were obtained when Co-protoporphyrin IX was used, with the difference that when alpha-tocopherol and allopurinol were given 2 h after administration of the inducer, they showed no protective effect. Phenylhydrazine and diamide also induced heme oxygenase activity in rat liver. This inductive effect was preceded by a decrease in the intrahepatic GSH pool, which took place several hours before induction of the oxygenase. Administration of alpha-tocopherol and allopurinol prevented induction of the oxygenase but had no effect on the decrease in GSH levels. These results suggest that the induction of heme oxygenase by phenylhydrazine and the diamide is preceded by an oxidative stress which very likely originates in the depletion of GSH. The induction of heme oxygenase by hemin was not prevented by administration of alpha-tocopherol or allopurinol. Coprotoporphyrin IX did not affect the pattern of the molecular forms of hepatic biliverdin reductase, at variance with CoCl2, which is known to convert molecular form 1 of the enzyme into molecular form 3.

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.

Modulation of insulin induced ornithine decarboxylase by putrescine and methylputrescines in H-35 hepatoma cells.

The effect of several methylputrescines on the activity of insulin-induced ornithine decarboxylase (ODC) was examined in H-35 hepatoma cells. The induction involved both protein and m-RNA synthesis. Actinomycin D inhibited ODC activity when given up to 1 h after insulin treatment. When added to the medium 2 h or 3 h after the insulin, the activity was increased 100% and 80% respectively. Insulin-induced ODC from H-35 cells had a biphasic half-life, a shorter one of 46 min and a longer one of 90 min. 1-Methylputrescine and 2-methylputrescine were found to be competitive inhibitors of the ODC from H-35 cells with Ki values of 2.8 and 0.1 mM respectively. Putrescine itself was found to have a Ki = 2.4 mM. N-Methylputrescine was a very poor inhibitor of the cell free ODC while 1,4-dimethylputrescine did not show any inhibitory effect. When cellular ODC activity was measured, the four methylputrescines assayed as well as putrescine entirely abolished its activity in the H-35 cells when given at a 1 mM concentration together with insulin. 1-Methylputrescine and 1,4-dimethylputrescine abolished 60% of the activity at a 0.1 microM concentration. All the methylputrescines given at 0.1 mM concentrations decreased the putrescine content of the stimulated cells to the levels found in quiescent cells, but only 1-methyl and 2-methylputrescines decreased spermidine and spermine content. 1,4-Dimethyl and 1-methylputrescines showed a strong inhibition of ODC synthesis, while the other diamines were less inhibitory. At concentrations that abolished ODC activity, 1,4-dimethylputrescine decreased 70% of the total immunoreactive ODC bands, while 1-methyl and 2-methylputrescine decreased them by 50%, and N-methylputrescine and putrescine decreased them by 20%. The lack of decrease in immuno-reactive ODC with the latter two compounds was mainly due to the appearance of immunoreactive degradation products of ODC of low molecular weight. Putrescine and N-methylputrescine affected protein synthesis to a small extent in stimulated cells, while 1-methylputrescine decreased it to the level of non-stimulated cells. Insulin (1 microM concentration) stimulated DNA synthesis in the cells, and this stimulation was doubled in the presence of 2-methylputrescine or putrescine. It can be concluded that, among the methylputrescines assayed, 2-methylputrescine was the best inhibitor of cell-free ODC activity, while 1,4-dimethylputrescine and 1-methylputrescine were the best inhibitors of cellular ODC activity.

A 13C NMR study of [5,8-13C2]spermidine binding to tRNA and to Escherichia coli macromolecules.

[5,8-13C2]Spermidine was prepared by synthesis, and its binding to macromolecular structures of Escherichia coli was studied. When added to E. coli cells, the two signals of [13C]spermidine (C-5, 47.8 ppm, and C-8, 39.6 ppm; JC-C = 5.8 Hz) were strongly broadened due to binding to macromolecules. When [13C]spermidine was added to E. coli tRNA, the C-5 resonance broadened to v1/2 = 4.7 Hz, whereas the C-8 resonance broadened to v1/2 = 2.7 Hz. tRNA-bound [13C]spermidine could be chased by [12C]spermidine or spermine, but not by putrescine or cadaverine. By using mixtures of [5-13C]- and [8-13C]spermidines (where 13C-13C coupling was avoided), it was possible to estimate a dissociation constant (Kd) of 3 x 10(-3) M using the C-5 v1/2obs values and a Kd of 2.10(-3) M using the C-8 v1/2obs values. The number of spermidine-binding sites (n) could also be estimated by fitting the bound spermidine molar fraction versus tRNA concentration. Values of n = 12 +/- 2 and 14 +/- 3 were obtained for C-5 and C-8, respectively. Measurements of line narrowing at increasing Mg2+ concentrations indicated that approximately 11 spermidines (of the 12-14 bound ones) could be displaced by the former, whereas 3 spermidines remain strongly bound to the tRNA backbone. Measurements of free and bound T1 allowed the determination of a correlation time of 10(-10)s for tRNA-bound spermidine.

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 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.

Inhibition of ornithine decarboxylase by the isomers of 1,4-dimethylputrescine.

1,4-Dimethylputrescine (2,5-hexanediamine) was separated into its racemic and meso isomers by fractional crystallization of its dibenzoyl derivative. The racemic form was resolved into its (+)- and (-)-isomers with (+)- and (-)-dibenzoyltartaric acids. None of the three isomers (meso, +, and -) inhibited ornithine decarboxylase (ODC) activity in vitro, while all the three were strongly inhibitory of ODC when assayed in vivo in rats or in H-35 hepatoma cells. In rat liver the three isomers also decreased the putrescine pool while only the (+)-isomer decreased spermidine content. In the H-35 cells the (-)- and (+)-isomers decreased the spermidine and spermine content. When ODC was induced in the latter by insulin it was found that the (-)-isomer strongly inhibited protein and ODC synthesis, while the (+)-isomer and the meso isomer were less inhibitory. The meso isomer was a good inducer of ODC antizyme in rat liver, while the (+)- and (-)-isomers were poor inducers of the former.

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.

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

Rat-liver biliverdin reductase exists in two molecular forms. The major form 1 has a molecular mass of 34 kDa, while the minor form 2 has a molecular mass of 56 kDa. Form 1 was converted into a second major form (form 3) with a molecular mass of 68 kDa by a NAD+-dependent peroxisomal dehydrogenase which was induced under conditions of oxidative stress [Frydman, R. B., Tomaro, M. L., Awruch, J. & Frydman, B. (1984) Biochem. Biophys. Res. Commun. 121, 249]. Molecular form 1 from rat kidney was not affected by the dehydrogenase, and a structural explanation for this difference was therefore sought. Both form 1 biliverdin reductases, isolated from rat liver and kidney, were purified to homogeneity using affinity chromatography, FPLC and HPLC techniques. The homogeneous enzymes were found to be identical when compared by their HPLC retention times, amino acid compositions and electrophoretic behaviour on polyacrylamide gels under non-denaturing conditions and on SDS/polyacrylamide gels. On HPLC analysis the peptides resulting from the CNBr cleavage were found to be the same for both enzymes, when either the native enzymes or their thioethylpyridine derivatives were compared. When the HPLC fingerprints of the tryptic digests were compared, they were found to be very similar, except for a peptide eluting at 31.60 min in the liver digest and at 23.60 min in the kidney digest. When the enzyme from both origins was alkylated with 4-dimethylaminoazobenzene-4'-iodoacetamide and then digested with trypsin, the HPLC fingerprints of the alkylated cysteine-carrying peptides were almost identical, except for a peptide with a retention time of 19.03 min in the liver digest and of 18.19 min in the kidney digest. The liver reductase was not amenable to Edman degradation suggesting a block at the NH2-terminus; in the kidney enzyme, however, it was free and an NH2-terminal sequence of 12 amino acids could be determined. The liver enzyme was found to be more sensitive toward p-hydroxymercuriphenyl sulfonate than the kidney enzyme.

Spectroscopic evidence for a porphobilinogen deaminase-tetrapyrrole complex that is an intermediate in the biosynthesis of uroporphyrinogen III.

Incubation of porphobilinogen (PBG) with PBG deaminase from Rhodopseudomonas sphaeroides in carbonate buffer (pH 9.2) to total PBG consumption resulted in low yields of uroporphyrinogen I (uro'gen I). In the reaction mixture a pyrrylmethane accumulated, which at longer incubation periods was transformed into uro'gen I. The accumulated pyrrylmethane gave an Ehrlich reaction which was different from that of a 2-(aminomethyl)dipyrrylmethane or 2-(aminomethyl)tripyrrane. It resembled that of a bilane (tetrapyrrylmethane) but was different from that of a 2-(hydroxymethyl)bilane. The 13C NMR spectra of incubations carried out with [11-13C]PBG indicated that the pyrrylmethane was a tetrapyrrole with methylene resonances at 22.35-22.50 ppm. It was loosely bound to the deaminase, and when separated from the enzyme by gel filtration or gel electrophoresis, it immediately cyclized to uro'gen I. No enzyme-bound methylene could be detected by its chemical shift, suggesting that its line width must be very broad. When uro'gen III-cosynthase was added to the deaminase-tetrapyrrole complex, uro'gen III was formed at the expense of the latter in about 75% yield. The tetrapyrrole could only be partially displaced from the enzyme by ammonium ions, although a small amount of 2-(aminomethyl)bilane was always formed together with the tetrapyrrole intermediate. A protonated uro'gen I structure for this intermediate was ruled out by incubations using [2,11-13C]PBG. Uro'gen III formation from 2-(hydroxymethyl)bilane (HMB) and from the deaminase-tetrapyrrole intermediate was compared by using deaminase-cosynthase and cosynthase from several sources. It was found that while the HMB inhibited uro'gen III formation at higher concentrations and longer incubation times, uro'gen III formation from the complex did not decrease with time.

Interaction of alkylputrescines with ornithine decarboxylase from rat liver and Escherichia coli: an in vitro and in vivo study.

The inhibitory effect of a series of 2-alkylputrescines on rat liver and Escherichia coli ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) was examined. At 2.5 mM concentrations, 2-methyl-, 2-propyl-, 2-butyl-, 2-pentyl- and 2-hexylputrescines were stronger inhibitors of the mammalian enzyme than putrescine. Only the higher homologues (from 2-propyl- to 2-hexylputrescine) were inhibitors of the E. coli enzyme. An analysis of the effect of increasing concentrations of the 2-alkylputrescines showed that the main difference in the behaviour of the mammalian and E. coli decarboxylases toward 2-alkylputrescines was that the former was strongly inhibited by 2-methylputrescine whereas the latter was not. 2-Alkylputrescines were found to be competitive inhibitors of both the bacterial and mammalian enzyme. The smallest Ki values (0.1 and 0.5 mM) were found for the 2-hexyl- and 2-pentylputresciens. N-Methyl-, N-ethyl-, N-propyl- and N-butylputrescines (50 mumol per 100 g body weight) were assayed as inhibitors of thioacetamide-induced rat liver ornithine decarboxylase. N-Propylputrescine was found to be the most inhibitory (66% inhibition) and although the N-alkylputrescines were taken up by the liver, they did not inhibit the liver polyamine pools. Both putrescine and N-methylputrescine were found to stabilize the thioacetamide-induced ornithine decarboxylase at the onset of the enzyme's degradation, while 2-alkylputrescines were inhibitory under similar conditions. N-Methylputrescine induced antizyme in thioacetamide-treated rats. In thioacetamide- or dexamethasone-treated rats, 2-methylputrescine was found to be the strongest in vivo inhibitor of the liver decarboxylase. Although 2-alkylputrescines were efficiently taken up by the liver, they did not noticeably inhibit its polyamine pools. 2-methylputrescine decreased the putrescine concentration of the liver, but not its spermidine and spermine content. No induction of ornithine decarboxylase antizyme by 2-methylputrescine could be detected. The intrahepatic concentration of the latter decreased with time, very likely due to its degradation by a diamine oxidase, since the decrease was inhibited by aminoguanidine.

Biliverdin reductase: substrate specificity and kinetics.

The substrate specificity of the different forms of rat liver biliverdin reductase was examined using synthetic biliverdins. Biliverdins carrying methyl, ethyl and one propionate residue in their structure were not substrates of biliverdin reductase. Biliverdins with one propionate and one acetate residue or with two acetate residues were not reduced by the enzyme either. The presence of two propionates in the biliverdin structure gave a biliverdin with substrate activity. Increasing the number of propionates to four, as in coprobiliverdins, did not affect substrate activity, while the octaacid urobiliverdins were also good substrates of the enzymes. The beta isomer of urobiliverdin III and coprobiliverdin III were reduced at much higher rates by molecular form 3 of the enzyme as compared to molecular form 1, a fact which had already been observed with the beta isomer of biliverdins IX, XIII and hematobiliverdin. All the biliverdins mentioned above were readily reduced to bilirubins by sodium borohydride. The purified molecular forms 1 and 3 displayed sigmoidal kinetics with most of the biliverdins tested. The data were analyzed by nonlinear regression in a microcomputer and it was found that they fitted a model of a moderate cooperative dimer where both ES and ES2 are catalytically active. The Vm, Ks and the Hill numbers, nH, for biliverdin IX alpha and beta, hematobiliverdin IX alpha and beta, and several synthetic biliverdin isomers are given. Molecular form 2 showed classical Michaelian kinetics.

Porphobilinogen oxygenase. Purification and evidence of its hemoprotein structure.

Porphobilinogen oxygenase oxidizes porphobilinogen to 2-hydroxy-5-oxo-porphobilinogen. This enzyme isolated from wheat germ has been purified to homogeneity, as judged by polyacrylamide gel electrophoresis under both nondenaturing and denaturing conditions. The molecular weight of the enzyme formed from two identical (or very similar) polypeptide chains is 70,000. It has a pI of 9.0 indicating its cationic nature. The pure enzyme contains 1 mol of high-spin heme and 2 mol of non-heme iron. It requires both of these as well as molecular O2 and a reducing agent for catalytic activity. Although the enzyme has many characteristics of a peroxidase, hydrogen peroxide cannot substitute for oxygen and dithionite for catalysis. The catalytic reaction is not affected by catalase, superoxide dismutase, or by hydroxyl radical scavengers. A comparison between porphobilinogen oxygenase and a commercial preparation of horseradish peroxidase was made. The latter also catalyzes aerobic porphobilinogen oxidation, with dithionite as electron donor. Here the oxidation of porphobilinogen is inhibited by superoxide dismutase and was not affected by catalase.