Differences in Functional Clustering of Endogenous and Exogenous Substrates Between Members of the CYP1A Subfamily

Biological Responses of Pacific Herring Embryos to Crude Oil Are Quantifiable at Exposure Levels Below Conventional Limits of Quantitation for PAHs in Water and Tissues

Pacific herring (Clupea pallasii), a cornerstone of marine food webs, generally spawn on marine macroalgae in shallow nearshore areas that are disproportionately at risk from oil spills. Herring embryos are also highly susceptible to toxicity from chemicals leaching from oil stranded in intertidal and subtidal zones. The water-soluble components of crude oil trigger an adverse outcome pathway that involves disruption of the physiological functions of cardiomyocytes in the embryonic herring heart. In previous studies, impaired ionoregulation (calcium and potassium cycling) in response to specific polycyclic aromatic hydrocarbons (PAHs) corresponds to lethal embryolarval heart failure or subtle chamber malformations at the high and low ends of the PAH exposure range, respectively. Sublethal cardiotoxicity, which involves an abnormal outgrowth (ballooning) of the cardiac ventricular chamber soon after hatching, subsequently compromises juvenile heart structure and function, leading to pathological hypertrophy of the ventricle and reduced individual fitness, measured as cardiorespiratory performance. Previous studies have not established a threshold for these sublethal and delayed-in-time effects, even with total (∑)PAH exposures as low as 29 ng/g of wet weight (tissue dose). Here, we extend these earlier findings showing that (1) cyp1a gene expression provides an oil exposure metric that is more sensitive than typical quantitation of PAHs via GC-MS and (2) heart morphometrics in herring embryos provide a similarly sensitive measure of toxic response. Early life stage injury to herring (impaired heart development) thus occurs below the quantitation limits for PAHs in both water and embryonic tissues as a conventional basis for assessing oil-induced losses to coastal marine ecosystems.

Altered substrate specificity of the Pterygoplichthys sp. (Loricariidae) CYP1A enzyme

Ethoxyresorufin is a classical substrate for vertebrate CYP1A enzymes. In Pterygoplichthys sp. (Loricariidae) this enzyme possesses 48 amino acids substitutions compared to CYP1A sequences from other vertebrate species. These substitutions or a certain subset substitution are responsible for the non-detection of the EROD reaction in this species liver microsomes. In the present study, we investigated the catalytic activity of Pterygoplichthys sp. CYP1A toward 15 potential substrates in order to understand the substrate preferences of this modified CYP1A. The fish gene was expressed in yeast and the accumulation of the protein was confirmed by both the characteristic P450-CO absorbance spectra and by detection with monoclonal antibodies. Catalytic activities were assayed with yeast microsomes and four resorufin ethers, six coumarin derivates, three flavones, resveratrol and ethoxyfluoresceinethylester. Results demonstrated that the initial velocity pattern of this enzyme for the resorufin derivatives is different from the one described for most vertebrate CYP1As. The initial velocity for the activity with the coumarin derivatives is several orders of magnitude higher than with the resorufins, i.e. the turnover number (kcat) for ECOD is 400× higher than for EROD. Nonetheless, the specificity constant (kcat/km) for EROD is only slightly higher than for ECOD. EFEE is degraded at a rate comparable to the resorufins. Pterygoplichthys sp. CYP1A also degrades 7-methoxyflavone and β-naphthoflavone but not resveratrol and chrysin. These results indicate a divergent substrate preference for Pterygoplichthys sp. CYP1A, which may be involved in the adaptation of Loricariidae fish to their particular environment and feeding habits.

Functional characterization of eight human CYP1A2 variants: the role of cytochrome b5

BACKGROUND

Interindividual variability in cytochrome P450 (CYP)-mediated xenobiotic metabolism is extensive. CYP metabolism requires two electrons, which can be donated by NADPH cytochrome P450 oxidoreductase (CYPOR) and/or cytochrome b5 (b5). Although substantial number of studies have reported on the function and effect of b5 in CYP-mediated catalysis, its mode of action is still not fully understood.

OBJECTIVE

The aim of this work was to examine the effect of b5 on the activities of eight natural-occurring variants of human CYP1A2, namely, T83M, S212C, S298R, G299S, I314V, I386F, C406Y, and R456H.

MATERIALS AND METHODS

An approach, as used in our former study was applied, coexpressing these polymorphic CYP1A2 variants separately with CYPOR and b5 in the bacterial cell model BTC-CYP. For each variant, 16 different activity parameters were measured, using eight different substrates. This heterogeneous data set was merged with the one of our former study (i.e. without b5) and a multivariate analysis was carried out.

RESULTS

This analysis indicated that b5 seems to have the ability to affect CYP1A2 variants to behave more like the wild-type variant. This was especially the case for variant I386F, for which the presence of b5 was crucial to show activity. Variants T83M and C406Y showed considerably different activity-profiles when in the presence of b5. Furthermore, our data seem to implicate CYP1A2 residue G299 in its interaction with CYPOR and/or b5.

CONCLUSION

Results indicate the ability of b5 to affect CYP1A2 variants to behave more like the wild-type variant, attenuating detrimental effects of structural mutations of these variants, seemingly through extensive allosteric effects.

Evaluation of the effects of 20 nonsynonymous single nucleotide polymorphisms of CYP2C19 on S-mephenytoin 4'-hydroxylation and omeprazole 5'-hydroxylation

CYP2C19 is a highly polymorphic enzyme that affects the metabolism of a wide range of therapeutic drugs. Almost all the identified alleles of CYP2C19 are derived from nonsynonymous single nucleotide polymorphisms (nsSNPs). The objective of this study was to functionally characterize 20 nsSNPs of CYP2C19, distributed throughout the entire coding region, most of which have not been thoroughly characterized. cDNAs of these variants were constructed and expressed in yeast cells. All variants had similar levels of apoprotein and holoprotein expression, except for CYP2C19.16 and D360N, which had significantly lower holoprotein levels than the wild-type (WT) CYP2C19 enzyme, and CYP2C19.5B, which showed only apoprotein. The activity of the CYP2C19 variants was investigated using two substrates, S-mephenytoin and omeprazole, and six different kinetic parameters were measured. CYP2C19.5B, CYP2C19.6, and CYP2C19.8 were found to be catalytically inactive. The entire dataset of the remaining 17 variants, together with the WT, was analyzed by multivariate analysis. This analysis indicated that CYP2C19.9, CYP2C19.10, CYP2C19.16, CYP2C19.18, CYP2C19.19, A161P, W212C, and D360N were substantially altered in catalytic properties in comparison with the WT, with each of these variants exhibiting either dramatically decreased catalytic activities or higher K(m) values. These results not only generally confirmed the function of previously reported variants but also identified additional reduced-function variants. These findings will greatly extend our understanding of CYP2C19 genetic polymorphisms in humans as well as facilitate the structure-function study of the CYP2C19 protein.

Cytochromes P450 are essential players in the vitamin D signaling system

From earliest development on, the vitamin D receptor (VDR) is expressed in most cells of the mammalian body. The VDR is a nuclear, ligand-induced transcription factor that regulates in complex with hormonally active vitamin D the expression of more than 900 genes involved in a wide array of physiological functions (e.g. calcium homeostasis, growth control, differentiation, cognition, immune response, etc.). Accordingly, severe health problems are associated to vitamin deficiencies. Synthesis of the major active form 1α,25(OH)₂D₃ from vitamin D and subsequent metabolism are exclusively controlled by specific P450-forms. Synthesis, a two-step process, starts with a 25-hydroxylation primarily by CYP2R1 (CYP27A1, CYP2J2, and CYP3A4 may also contribute) and a subsequent 1α-hydroxylation via CYP27B1. Circulating in the bloodstream, 1α,25(OH)₂D₃ acts at sites of VDR expression (target sites) in an endocrine way. However, it is also capable of autocrine/paracrine functions since various target tissues are fully competent in 1α,25(OH)₂D₃ synthesis, as illustrated by three examples. 1α,25(OH)₂D₃ levels are short-lived: the hormone upregulates its rapid metabolism by CYP24A1 that attacks repeatedly the vitamin D C₂₀₋₂₇ side chain, thereby producing a complex cascade of transient metabolites with increasing polarity. Most of these metabolites still retain 1α,25(OH)₂D₃-like activities on the VDR, contributing to the overall effect that is commonly attributed to 1α,25(OH)₂D₃. As selective inhibitors of CYP24A1 increase the lifetime and thereby the function of vitamin D metabolites, they will help exploring whether and which intrinsic activities distinct metabolites possess. It appears likely that this strategy may unmask important regulators of new functions.