JUD COON: 35 YEARS OF P450 RESEARCH, A SYNOPSIS OF P450 HISTORY

Organ-specific and genotype-dependent constitutive biosynthesis of secoiridoid glucosides in Centaurium erythraea Rafn, and its elicitation with methyl jasmonate

While bioactive properties of Centaurium erythraea Rafn secoiridoid glucosides (SG) are widely recognized, many aspects related to their biochemistry, metabolism and relationship to the overall plant physiology are not yet understood. Here we present for the first time an insight into the molecular background of organ-specific and genotype-dependent constitutive biosynthesis of secoiridoids in C. erythraea, by comparing chemical profiles and secoiridoid glucosides-related gene expression. Genes encoding enzymes for intermediate steps of secoiridoids biosynthesis up to secologanin have been identified by analysing transcriptomic data from C. erythraea leaves. Results suggest an organ-specific capacity for the production and accumulation of secoiridoid glucosides, and highlight leaves as the main biosynthesis site. They also point out that significant differences in SG content among various C. erythraea genotypes, are, at least partially, determined by different expression patterns of SG-related genes. The biosynthesis of SG in C. erythraea leaves is enhanced upon treatments with methyl jasmonate (MeJA), which causes reprogramming of SG-related gene expression, leading to an increased production of valuable bioactive compounds. The present study unveiled several rate-limiting genes (encoding GES, G8O, 8HGO, IS and 7DLGT) in SG biosynthesis. SLS and CPR are highlighted as important genes/enzymes that might regulate biosynthetic flux through SG pathway. Information gathered within this study will help us gain deeper insight into the SG metabolism and develop strategies for enhanced biosynthesis of specific secoiridoid glucosides in homologous or heterologous systems.

Allele and genotype frequencies of cytochrome P450 2B6 gene in a Mongolian population

CYP2B6 plays an important role in metabolizing various drugs in common clinical use. Increasing interest in CYP2B6 genetic polymorphism was stimulated by revelations of a specific CYP2B6 genotype significantly affecting the metabolism of efavirenz, an anti-HIV type-1 agent. The present study determined the CYP2B6 haplotype in 100 healthy unrelated Mongolian volunteers by analyzing the genotypes of nine single nucleotide polymorphism (SNP) positions (-82T>C, 64C>T, 499C>T, 516G>T, 777C>A, 785A>G, 983T>C, 1375A>G, and 1459C>T) in the CYP2B6 gene. The CYP2B6 *1 allele was the most frequent in the Mongolian population tested at 64.5%, higher than the equivalent frequency in African-Americans and Ghanaians. The second most frequent allele was CYP2B6 *6 (21.0%), although this allele was less frequent than that in Ghanaians. Only one CYP2B6 *5 allele was identified in our Mongolian subjects (0.5%), although it is the third most frequent allele in white and African-American populations. These CYP2B6 genotypes revealed seven slow efavirenz metabolizers in 100 Mongolians, which is significantly fewer than the same group among Ghanaians. Overall, the Mongolian CYP2B6 allele distribution was comparable with that in Japanese, Koreans, and Han Chinese. This is the first report of CYP2B6 genotype frequency in a Mongolian population, and it could provide clinically useful information on drug metabolism in this population group.

Purification, cDNA cloning and functional expression of NADPH-cytochrome P450 reductase from Centaurium erythraea cell cultures

Solubilised NADPH-cytochrome P450 reductase (CPR) was purified from the microsomal fraction of centaury (Centaurium erythraea) cell cultures by Q-anion exchange chromatography and affinity chromatography on adenosine 2',5'-diphosphate agarose. SDS-PAGE demonstrated the presence of three CPR isoforms with molecular masses of 77, 79 and 81 kDa. The 79- and 81-kDa isoforms were identified as glycoproteins when blotted following SDS-PAGE and subjected to a sugar detection procedure. A homology-based approach led to the isolation of a CPR cDNA encoding the 77-kDa isoform. The enzyme was a class I CPR, possessing a short N-terminus upstream of the membrane anchor. The amino acid sequence contained a putative N-glycosylation site, indicating that the two major isoforms of 77 and 79 kDa are related through attachment of an oligosaccharide chain. This glycosylation process was also found upon heterologous expression in yeast. When co-expressed in yeast together with centaury coniferyl alcohol 5-hydroxylase, CPR efficiently supported the activity of the P450 enzyme. The genome of C. erythraea was found to contain a second CPR gene. RT-PCR experiments using gene-specific primers revealed differential regulation of the two CPR genes. While CPR 2 mRNA was strongly induced by the addition of methyl jasmonate to the cell cultures, the CPR 1 expression level did not change after this elicitation.

cDNA cloning and functional characterisation of CYP98A14 and NADPH:cytochrome P450 reductase from Coleus blumei involved in rosmarinic acid biosynthesis

The final reactions of rosmarinic acid biosynthesis, the introduction of the aromatic 3- and 3'-hydroxyl groups, are catalysed by cytochrome P450-dependent hydroxylases. The cDNAs encoding CYP98A14 as well as a NADPH:cytochrome P450 reductase (CPR) were isolated from Coleus blumei and actively expressed in Saccharomyces cerevisiae. The CYP98A14-cDNA showed an open reading frame of 1521 nucleotides with high similarities to 4-coumaroylshikimate/quinate 3-hydroxylases. Yeast microsomes harbouring the CYP98A14 protein catalysed the 3-hydroxylation of 4-coumaroyl-3',4'-dihydroxyphenyllactate and the 3'-hydroxylation of caffeoyl-4'-hydroxyphenyllactate, in both cases forming rosmarinic acid. Apparent K (m)-values for 4-coumaroyl-3',4'-dihydroxyphenyllactate and caffeoyl-4'-hydroxyphenyllactate were determined to be at 5 microM and 40 microM, respectively. CYP98A14 differs from CYP98s from other plants, since 4-coumaroylshikimate or -quinate were not accepted as substrates. Coexpression of the Coleus blumei CPR and CYP98A14 in the same yeast cells increased the hydroxylation activity up to sevenfold. CYP98A14 from Coleus blumei is a novel bifunctional cytochrome P450 specialised for rosmarinic acid biosynthesis.

Stereoselective disposition of fluoxetine and norfluoxetine during pregnancy and breast-feeding

AIMS

To compare the disposition of fluoxetine and norfluoxetine enantiomers in the mother, foetus and infant.

METHODS

Blood from pregnant women taking fluoxetine (n = 9), during pregnancy was sampled in the third trimester and at delivery (maternal and cord venous blood), and from the infants 48 h after delivery. The subset of these women who were breastfeeding, plus additional subjects recruited in the postpartum period, were studied further, and maternal and infant blood, and breast milk was sampled between 6 days and 11 months (n = 23). Drug and metabolite concentrations were measured using gas chromatography/mass spectrometry or liquid chromatography, tandem mass spectrometry.

RESULTS

There was a high correlation between maternal and foetal (cord blood) fluoxetine and norfluoxetine enantiomers (r(2)-0.9), the mean foetal/maternal ratios (95% confidence intervals) being 0.91 (0.61, 1.02) and 1.04 (0.93, 1.05), for fluoxetine and norfluoxetine, respectively. In 2 day old infants exposed to the drug in utero, the fluoxetine and norfluoxetine plasma concentrations were the same as in cord blood at delivery. Over the next 2 months, the plasma concentrations in the infants fell progressively. Stereoselective disposition of both the drug and metabolite in the mother, foetus, infant and breast milk was observed. The S : R ratios in the foetus and newborn ( approximately 3) were significantly higher than in the serum ( approximately 2) or breast milk ( approximately 1.9) of the mothers, resulting in greater exposure of the foetus and infants to the biologically active enantiomers, particularly S-norfluoxetine.

CONCLUSIONS

Foetal and infant exposure to fluoxetine and norfluoxetine is enhanced by their stereoselective disposition in the mother, foetus, breast milk and infant. Increased exposure may also result from decreased metabolism of the drug in the foetus and neonate.

Identification of a novel specific CYP2B6 allele in Africans causing impaired metabolism of the HIV drug efavirenz

The non-nucleoside reverse transcriptase inhibitor efavirenz is mainly metabolised by the polymorphic cytochrome P450 enzyme CYP2B6. Genomic DNA from four subjects in a group of 51 patients being treated with efavirenz and having surprisingly high plasma concentrations were screened by direct sequencing for mutations in the CYP2B6 gene. Four exonic single nucleotide polymorphisms (SNPs), 516G > T, 714G > A, 785A > G and 983T > C, and eight intronic SNPs were identified. Haplotype analysis revealed that 983T > C was linked with 785A > G defining a novel allele, CYP2B6*16. This allele was present in totally five of the patients. The CYP2B6.16 cDNA was expressed in yeast and HEK293 cells and significantly less protein was formed compared to the wild-type cDNA, in both heterologous systems. By contrast, the catalytic activity of the enzyme variant was not different from the CYP2B6.1 enzyme, using bupropion as a probe substrate. The CYP2B6*16 allele was not found in Swedes, was present at 4% frequency among Turks, but was common among Africans. The steady-state level of efavirenz was significantly higher in the five carriers of CYP2B6*16, being of African origin, compared to the other patients. Higher efavirenz concentrations were also seen in carriers of 516G>T (CYP2B6*6 and CYP2B6*9). In conclusion, a novel CYP2B6*16 allele causing less expression of the corresponding enzyme was identified and found to influence the metabolism of efavirenz in vivo, a finding that is of potential impact for anti-HIV therapy in black populations.

CYTOCHROME P450: Nature's Most Versatile Biological Catalyst

The author describes studies that led to the resolution and reconstitution of the cytochrome P450 enzyme system in microsomal membranes. The review indicates how purification and characterization of the cytochromes led to rigorous evidence for multiple isoforms of the oxygenases with distinct chemical and physical properties and different but somewhat overlapping substrate specificities. Present knowledge of the individual steps in the P450 and reductase reaction cycles is summarized, including evidence for the generation of multiple functional oxidants that may contribute to the exceptional diversity of the reactions catalyzed.

Early years of oxygenase research in Bethesda, Osaka, Urbana, and Kanazawa

In this brief review, I recollect my experiences of how the studies of pyrocatechase and salicylate hydroxylase led to the isolation and revelation of P450cam. Those experiences were instrumental in the separation, purification, and characterization of the two forms of adrenal cortex mitochondrial P450.

Mechanism-Based Inactivation of CYP3A by HIV Protease Inhibitors

Human immunodeficiency virus (HIV) protease inhibitors (PIs) are inhibitors of CYP3A enzymes, but the mechanism is poorly defined. In this study, time- and concentration-dependent decreases in activity as defined by maximum rate of inactivation (k(inact)) and inhibitor concentration that gives 50% maximal inactivation (K(I)) of CYP3A by amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir were quantified using testosterone 6beta-hydroxylation as a marker for CYP3A activity with recombinant CYP3A4(+b(5)), recombinant CYP3A5, and pooled human liver microsomes (HLMs). All the PIs, except indinavir, displayed inactivation with CYP3A4(+b(5)) and HLMs. Ritonavir was the most potent (K(I) = 0.10 and 0.17 microM) and demonstrated high k(inact) values (0.32 and 0.40 min(-1)) with both CYP3A4(+b(5)) and HLMs. Ritonavir was not significantly depleted by high-affinity binding with CYP3A4(+b(5)) and confirmed that estimation of reversible inhibition was confounded with irreversible inhibition. For CYP3A5, nelfinavir exhibited the highest k(inact) (0.47 min(-1)), but ritonavir was the most potent (K(I) = 0.12 microM). Saquinavir and indinavir did not show time- and concentration-dependent decreases in activity with CYP3A5. Spectrophototmetrically determined metabolic intermediate complex formation was observed for all of the PIs with CYP3A4(+b(5)), except for lopinavir and saquinavir. The addition of nucleophilic and free aldehyde trapping agents and free iron and reactive oxygen species scavengers did not prevent inactivation of CYP3A4(+b(5)) by ritonavir, amprenavir, or nelfinavir, but glutathione decreased the inactivation by saquinavir (17%) and catalase decreased the inactivation by lopinavir (39%). In conclusion, all the PIs exhibited mechanism-based inactivation, and predictions of the extent and time course of drug interactions with PIs could be underestimated if based solely on reversible inhibition.