| 1. |
- Abu-Bakar, A'edah, et al.
(author)
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Inducible bilirubin oxidase : A novel function for the mouse cytochrome P450 2A5
- 2011
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In: Toxicology and Applied Pharmacology. - : Elsevier BV. - 0041-008X .- 1096-0333. ; 257:1, s. 14-22
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Journal article (peer-reviewed)abstract
- We have previously shown that bilirubin (BR), a breakdown product of haem, is a strong inhibitor and a high affinity substrate of the mouse cytochrome P450 2A5 (CYP2A5). The antioxidant BR, which is cytotoxic at high concentrations, is potentially useful in cellular protection against oxygen radicals if its intracellular levels can be strictly controlled. The mechanisms that regulate cellular BR levels are still obscure. In this paper we provide preliminary evidence for a novel function of CYP2A5 as hepatic "BR oxidase''. A high-performance liquid chromatography/electrospray ionisation mass spectrometry screening showed that recombinant yeast microsomes expressing the CYP2A5 oxidise BR to biliverdin, as the main metabolite, and to three other smaller products with m/z values of 301,315 and 333. The metabolic profile is significantly different from that of chemical oxidation of BR. In chemical oxidation the smaller products were the main metabolites. This suggests that the enzymatic reaction is selective, towards biliverdin production. Bilirubin treatment of primary hepatocytes increased the CYP2A5 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A5 compared to cells treated only with CHX. Collectively, the observations suggest that the CYP2A5 is potentially an inducible "BR oxidase" where BR may accelerate its own metabolism through stabilization of the CYP2A5 protein. It is possible that this metabolic pathway is potentially part of the machinery controlling intracellular BR levels in transient oxidative stress situations, in which high amounts of BR are produced.
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| 2. |
- Abu-Bakar, A'edah, et al.
(author)
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Metabolism of bilirubin by human cytochrome P450 2A6
- 2012
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In: Toxicology and Applied Pharmacology. - : Elsevier BV. - 0041-008X .- 1096-0333. ; 261:1, s. 50-58
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Journal article (peer-reviewed)abstract
- The mouse cytochrome P450 (CYP) 2A5 has recently been shown to function as hepatic "Bilirubin Oxidase" (Abu-Bakar, A., et al., 2011. Toxicol. Appl. Pharmacol. 257, 14-22). To date, no information is available on human CYP isoforms involvement in bilirubin metabolism. In this paper we provide novel evidence for human CYP2A6 metabolising the tetrapyrrole bilirubin. Incubation of bilirubin with recombinant yeast microsomes expressing the CYP2A6 showed that bilirubin inhibited CYP2A6-dependent coumarin 7-hydroxylase activity to almost 100% with an estimated K-i of 2.231 mu M. Metabolite screening by a high-performance liquid chromatography/electrospray ionisation mass spectrometry indicated that CYP2A6 oxidised bilirubin to biliverdin and to three other smaller products with m/z values of 301,315 and 333. Molecular docking analyses indicated that bilirubin and its positively charged intermediate interacted with key amino acid residues at the enzyme's active site. They were stabilised at the site in a conformation favouring biliverdin formation. By contrast, the end product, biliverdin was less fitting to the active site with the critical central methylene bridge distanced from the CYP2A6 haem iron facilitating its release. Furthermore, bilirubin treatment of HepG2 cells increased the CYP2A6 protein and activity levels with no effect on the corresponding mRNA. Co-treatment with cycloheximide (CHX), a protein synthesis inhibitor, resulted in increased half-life of the CYP2A6 compared to cells treated only with CHX. Collectively, the observations indicate that the CYP2A6 may function as human "Bilirubin Oxidase" where bilirubin is potentially a substrate and a regulator of the enzyme.
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| 3. |
- Richardson, James E., et al.
(author)
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The influence of tectonics, sea-level changes and dispersal on migration and diversification of Isonandreae (Sapotaceae)
- 2014
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In: Botanical journal of the Linnean Society. - : Wiley-Blackwell. - 0024-4074 .- 1095-8339. ; 174:1, s. 130-140
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Journal article (peer-reviewed)abstract
- Internal transcribed spacer (ITS) ribosomal DNA sequence data were generated for 80 of the c. 200 species of Isonandreae and were added to data from African and Neotropical representatives in subfamily Sapotoideae and outgroups in Sapotaceae. Bayesian dating and ancestral area reconstruction indicated that Isonandreae are derived from within an African grade. Multiple Australasian species or lineages are derived from Sundanian lineages in South-East Asia with stem ages originating from the late Oligocene. Sri Lankan and Indian lineages are also derived from Sundanian lineages. Our results are consistent with migration from Africa into Sundania followed by numerous over-water dispersal events across Wallace's Line into Australasia and migration from Sundania to the Indian subcontinent. Pleistocene speciation indicates that sea-level changes during that epoch could have been responsible for some species diversification in Sundania.
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