The influence of side chain modifications of the heme moiety on prosthetic acceptance and function of rat hepatic cytochrome P-450 and tryptophan pyrrolase

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

Heme is vital to our aerobic universe. Heme cellular content is finely tuned through an exquisite control of synthesis and degradation. Heme deficiency is deleterious to cells, whereas excess heme is toxic. Most of the cellular heme serves as the prosthetic moiety of functionally diverse hemoproteins, including cytochromes P450 (P450s). In the liver, P450s are its major consumers, with >50% of hepatic heme committed to their synthesis. Prosthetic heme is the sine qua non of P450 catalytic biotransformation of both endo- and xenobiotics. This well-recognized functional role notwithstanding, heme also regulates P450 protein synthesis, assembly, repair, and disposal. These less well-appreciated aspects are reviewed herein.

Cobalt protoporphyrin inhibition of lipopolysaccharide or lipoteichoic acid-induced nitric oxide production via blocking c-Jun N-terminal kinase activation and nitric oxide enzyme activity

In the present study, low doses (0.5, 1, and 2 microM) of cobalt protoporphyrin (CoPP), but not ferric protoporphyrin (FePP) or tin protoporphyrin (SnPP), significantly inhibited lipopolysaccharide (LPS) or lipoteichoic acid (LTA)-induced inducible nitric oxide (iNOS) and nitric oxide (NO) production with an increase in heme oxygenase 1 (HO-1) protein in RAW264.7 macrophages under serum-free conditions. IC(50) values of CoPP inhibition of NO and iNOS protein individually induced by LPS and LTA were around 0.25 and 1.7 microM, respectively. This suggests that CoPP is more sensitive at inhibiting NO production than iNOS protein in response to separate LPS and LTA stimulation. NO inhibition and HO-1 induction by CoPP were blocked by the separate addition of fetal bovine serum (FBS) and bovine serum albumin (BSA). Decreasing iNOS/NO production and increasing HO-1 protein by CoPP were observed with CoPP pretreatment, CoPP co-treatment, and CoPP post-treatment with LPS and LTA stimulation. LPS- and LTA-induced NOS/NO productions were significantly suppressed by the JNK inhibitor, SP600125, but not by the ERK inhibitor, PD98059, through a reduction in JNK protein phosphorylation. Transfection of a dominant negative JNK plasmid inhibited LPS- and LTA-induced iNOS/NO production and JNK protein phosphorylation, suggesting that JNK activation is involved in LPS- and LTA-induced iNOS/NO production. Additionally, CoPP inhibition of LPS- and LTA-induced JNK, but not ERK, protein phosphorylation was identified in RAW264.7 cells. Furthermore, CoPP significantly reduced NO production in a cell-mediated, but not cell-free, iNOS enzyme activity assay accompanied by HO-1 induction. However, attenuation of HO-1 protein stimulated by CoPP via transfection of HO-1 siRNA did not affect NO's inhibition of CoPP against LPS stimulation. CoPP effectively suppressing LPS- and LTA-induced iNOS/NO production through blocking JNK activation and iNOS enzyme activity via a HO-1 independent manner is first demonstrated herein.

Assembly and activation of heme-deficient neuronal NO synthase with various porphyrins

The heme prosthetic group of NO synthase is critical for catalytic activity as well as assembly of the enzyme to the native homodimeric form. In the current study, we examined if structurally different metal porphyrins could substitute for the native heme prosthetic group in neuronal NO synthase (nNOS) with regard to assembly and catalysis. We established, with the use of a recently developed in vitro method that functionally reconstitutes heme-deficient apo-nNOS, that Fe-mesoporphyrin IX or Fe-deuteroporphyrin IX can substitute for heme and lead to assembly of a functional nNOS, albeit with lower activity. Fe-protoporphyrin IX dimethyl ester or the metal free protoporphyrin IX, however, lead to minimal assembly of nNOS. Protoporphyrin IX compounds where the native Fe was substituted with Zn, Mn, Co, or Sn lead to assembly of nNOS, but no detectable NO was synthesized in the presence of NADPH and L-arginine. Thus, the presence of the metal and propionic acid groups, but not the vinyl moieties, of heme are important structural features in assembly of nNOS. These studies establish that the mechanism of assembly and catalysis of nNOS can be probed with structurally diverse metal porphyrins.

Secobarbital-mediated inactivation of cytochrome P450 2B1 and its active site mutants. Partitioning between heme and protein alkylation and epoxidation

Secobarbital (SB) is a relatively selective mechanism-based inactivator of cytochrome P450 2B1, that partitions between epoxidation and heme and protein modification during its enzyme inactivation. The SB-2B1 heme adduct formed in situ in a functionally reconstituted system has been spectrally documented and structurally characterized as N-(5-(2-hydroxypropyl)-5-(1-methylbutyl)barbituric acid)protoporphyrin IX. The SB-protein modification has been localized to 2B1 peptide 277-323 corresponding to the active site helix I of cytochrome P450 101. The targeting of heme and this active site peptide suggests that the 2B1 active site topology could influence the course of its inactivation. To explore this possibility, the individual SB epoxidation, heme and protein modification, and corresponding molar partition ratios of the wild type and seven structural 2B1 mutants, site-directed at specific substrate recognition sites, and known to influence 2B1 catalysis were examined after Escherichia coli expression. These studies reveal that Thr-302 is critical for SB-mediated heme N-alkylation, whereas Val-367 is a critical determinant of 2B1 protein modification, and Val-363 is important for SB epoxidation. SB docking into a refined 2B1 homology model coupled with molecular dynamics analyses provide a logical rationale for these findings.

Loss and degradation of enzyme-bound heme induced by cellular nitric oxide synthesis

We report here that, like nonheme iron, protein-bound intracellular heme iron is also a target for destruction by endogenously produced nitric oxide (NO). In isolated rat hepatocytes NO synthesis results in substantial (approximately 60%) and comparable loss of catalase and cytochrome P450 as well as total microsomal heme, and decreased heme synthetic (delta-aminolevulinate synthetase and ferrochelatase) and increased degradative (heme oxygenase) enzymatic activities. The effect is reversible, and intact cytochrome P450 apoproteins are still present, as judged by heme reconstitution of isolated microsomes. The effects on delta-aminolevulinate synthetase and heme oxygenase are likely to be secondary to heme liberation, while the effects on ferrochelatase appear to be a direct effect of NO, perhaps destruction of its nonheme iron-sulfur center.

Nitric oxide and intracellular heme

Figure 2 depicts a working hypothesis for these results. Activation of .NO synthesis results in nitrogen oxide-induced loss of protein-bound heme from CYP proteins, which remain relatively intact. This heme liberation results in a decrease in heme synthesis (decreased ALAS) and an increase in heme degradation (increased HO). In addition, .NO synthesis results in direct inhibition of ferrochelatase, which further contributes to inhibition of heme synthesis. There also appears to be a mechanism to repair or resynthesize CYP after .NO synthesis is inhibited. Finally, a result of this effect may be protection against cellular injury, since increased HO is an important response against cellular injury from a variety of insults.

Pharmaceutics and drug delivery aspects of heme and porphyrin therapy

The importance of porphyrins and metalloporphyrins as therapeutic drugs has increased significantly over the last decade. This review highlights some of the challenges faced by pharmaceutical scientists in formulating these drugs into stable, effective, and safe dosage forms. Most activity in the clinic has focused on three areas: photodynamic therapy of cancer (e.g., hematoporphyrin derivatives), porphyrias and hematological diseases (e.g., heme), and various forms of jaundice (e.g., tin porphyrins). The biodistribution, stability, aggregation, toxicology, and analytical methodology of porphyrin drugs are all important considerations in the pharmaceutical development of porphyrin drugs. The utility of delivery systems such as liposomes hold promise of increasing the therapeutic potential of these drugs. Future prospects for therapeutic applications of porphyrin drugs are also discussed.

Protein-mediated efflux of heme from isolated rat liver mitochondria

Proteins are required for the efflux of heme from mitochondria and liposomes. The efflux from liposomes is independent of the heme-binding affinity of the protein (Biochem. 23:3715, 1984). We tested whether heme-binding proteins increase efflux of newly synthesized heme from structurally and functionally intact rat liver mitochondria. Mitochondria whose heme was labeled with 14C-delta-aminolevulinic acid, were incubated in the presence of glutathione transferases (GSTs), serum albumin (RSA) or heme-binding protein (HBP), all from the rat. HBP caused a 6-8 fold increase in efflux of newly synthesized heme as compared to that effected by RSA or GSTs. This result indicates that heme efflux from intact mitochondria, unlike that from liposomes, depends on the type of protein present and that HBP may specifically facilitate heme efflux from mitochondria.

Novel exogenous heme-dependent expression of mammalian cytochrome P450 using baculovirus

Rat P450 IIA1 is a membrane-bound hemoprotein that catalyzes the 7 alpha- and 6 alpha-hydroxylation of testosterone. A recombinant baculovirus, containing the cDNA encoding IIA1, was constructed and used to infect Spodoptera frugiperda cells. Infected cells contained up to 10% IIA1 protein as quantified by Western blot analysis; however, only a small percentage of this protein contained heme and was catalytically active. Addition of hemin to the culture medium during the course of viral infection resulted in a marked sixfold increase in both testosterone hydroxylase activity and heme-containing IIA1 protein. When the exogenously added protoheme was substituted with modified heme molecules containing hydrogen or ethyl groups at the 2 and 4 positions of the protoporphyrin IX, enzymes with only marginal changes in catalytic properties were produced. These data indicate that baculovirus can be used as a system to produce high levels of mammalian cytochrome P450s containing custom modified heme moieties.