Structure-function analysis of CYP2A10 and CYP2A11, P450 cytochromes that differ in only eight amino acids but have strikingly different activities toward testosterone and coumarin

Lipidomics combined with transcriptomic and mass spectrometry imaging analysis of the Asiatic toad (Bufo gargarizans) during metamorphosis and bufadienolide accumulation

Background

To adapt to life on land, Asiatic toads (Bufo gargarizans) must remodel their bodies and refine their chemical defenses in water. The full scope of the mechanisms underlying these processes has yet to be revealed. Bufadienolides (BDs) are chemical defense substances secreted by toads when they are in danger, and they have high medicinal value in treating heart failure, cancer, and hepatitis. However, the artificial breeding of toads to increase BDs has been unsuccessful due to the high mortality of toad larvae during metamorphosis.

Method

Toad larvae at different growth stages were selected to study the changes in the metamorphosis process under the same growth conditions. The differences of tadpoles were explored, including body remodeling, energy metabolism, synthesis and regulation of BDs, through lipidomic technology, transcriptomic technology, and mass spectrometry imaging technology during metamorphosis.

Results

During metamorphosis, tadpoles underwent significant changes in lipid metabolism due to body remodeling to adapt to terrestrial life, which involved ketosis, lipogenesis, cholesterol metabolism, and fatty acid oxidation. The accumulation trend of BDs was observed. “Pentose phosphate pathway” and “Aromatase activity” may be the critical pathway and GO term in BD synthesis, involving 16 genes predominantly expressed in the liver. The involved genes were mainly expressed in the liver, consistent with the synthetic site observed by mass spectrometry imaging.

Conclusion

Together, our findings presented the changes in the toad larvae during metamorphosis and highlighted the accumulation process of BDs as well as the regulatory pathways and synthetic site, providing research and theoretical basis for future development of the toad resources.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13020-022-00676-7.

Bioactivation of coumarin in rat olfactory mucosal microsomes: Detection of protein covalent binding and identification of reactive intermediates through analysis of glutathione adducts

The presence of high levels, as well as tissue-specific forms, of cytochrome P450 enzymes in mammalian olfactory mucosa (OM) has important implications in the bioactivation and toxicity of xenobiotics entering the tissue. Previous studies have shown that coumarin, a known olfactory toxicant in rats, is bioactivated by OM microsomal P450s to a number of products, presumably via coumarin-3,4-epoxide and other epoxide intermediates. The aim of the current study was to obtain direct evidence for the formation of such reactive intermediates in rat OM through the detection of protein covalent binding and glutathione (GSH) adduct formation. Protein covalent binding experiments with [(14)C]coumarin (10microM) displayed a 7-9-fold higher NADPH-dependent radioactivity binding in rat OM microsomes (2.5nmol/mg/30min) compared to those in rat and human liver microsomes; the binding value in rat OM microsomes was substantially but not completely reduced by the addition of GSH (5mM). LC/MS analyses detected a number of GSH adducts in GSH-supplemented coumarin metabolism reaction in rat OM microsomes; 3-glutathionyl coumarin was found to be the major one, indicating 3,4-epoxidation as the main bioactivation pathway. Additional GSH adducts were identified, presumably forming via the same pathway or epoxidation on the benzene moiety. Our findings provide direct evidence for the formation of multiple coumarin reactive intermediates in rat OM, leading to protein covalent binding and GSH conjugation.

Identification of Val117 and Arg372 as critical amino acid residues for the activity difference between human CYP2A6 and CYP2A13 in coumarin 7-hydroxylation

Human cytochrome P450 (CYP) 2A6 and 2A13 play an important role in catalyzing the metabolism of many environmental chemicals including coumarin, nicotine, and several tobacco-specific carcinogens. Both CYP2A6 and CYP2A13 proteins are composed of 494 amino acid residues. Although CYP2A13 shares a 93.5% identity with CYP2A6 in the amino acid sequence, it is only about one-tenth as active as CYP2A6 in catalyzing coumarin 7-hydroxylation. To identify the key amino acid residues that account for such a remarkable difference, we generated a series of CYP2A6 and CYP2A13 mutants by site-directed mutagenesis/heterologous expression and compared their coumarin 7-hydroxylation activities. In CYP2A6, the amino acid residues at position 117 and 372 are valine (Val) and arginine (Arg), respectively; whereas in CYP2A13, they are alanine (Ala) and histidine (His). Kinetic analysis revealed that the catalytic efficiency (Vmax/Km) of the CYP2A6 Val(117)--> Ala and Arg(372)--> His mutants was drastically reduced (0.41 and 0.64 versus 3.23 for the wild-type CYP2A6 protein). In contrast, the catalytic efficiency of the CYP2A13 Ala(117) --> Val and His(372) --> Arg mutants was greatly increased (2.65 and 2.60 versus 0.31 for wild-type CYP2A13 protein). These results clearly demonstrate that the Val at position 117 and Arg at position 372 are critical amino acid residues for coumarin 7-hydroxylation. Based on the crystal structure of CYP2C5, we have generated the homology models of CYP2A6 and CYP2A13 and docked the substrate coumarin to the active site. Together with the kinetic characterization, our structural modeling provides explanations for the amino acid substitution results and the insights of detailed enzyme-substrate interactions.

Expression of cytochrome P450 2A3 in rat esophagus: relevance to N-nitrosobenzylmethylamine

N-nitrosobenzylmethylamine (NBzMA) must be metabolically activated to exert its carcinogenic potential and is a potent inducer of tumors in the rat esophagus. The activation is believed to occur in the esophagus. Although the pathways of NBzMA metabolism are well studied, the principal cytochrome P450 enzyme(s) (P450) responsible for catalyzing its activation is unknown. Several preliminary studies have suggested that this enzyme may belong to the P450 2A family. We report here that P450 2A3 expressed in a baculovirus system metabolizes NBzMA, predominantly by methylene hydroxylation. To determine whether or not P450 2A3 is present in the rat esophagus, the relative level of P450 2A3 mRNA was determined by reverse transcriptase-polymerase chain reaction (RT-PCR). The mRNA levels of P450 2A3 were compared with the levels of P450 2A1 and 2A2 mRNA in the esophagus, liver, lung and nasal mucosa. P450 2A3 mRNA was detected in rat nasal mucosa, lung and esophagus, but not in liver, whereas P450 2A1 and 2A2 mRNAs were detected only in the liver. To determine the relative expression of P450 2A3 in each tissue, quantitative RT-PCR with PCR-MIMICS used as internal standards was performed. The expression level in the nasal mucosa was by far the greatest. The expression in the lung and esophagus was 60- and 1600-fold less, respectively. Using antibodies to P450 2A4/5 and P450 2A10/11 a 50 kDa immunoreactive protein was detected in all three tissues by western blot analysis. This is consistent with the expression of P450 2A3 in these tissues. However, the amount of protein detected in the nasal mucosa was much greater than that in the esophagus or lung. The expression of P450 2A protein was similar in the lung and esophagus. The rate of coumarin 7-hydroxylation in cultured rat esophagus was very low. This is a reaction efficiently catalyzed by P450 2A5, 2A6 and 2A10. In summary, our results clearly demonstrate the presence of P450 2A3 protein and mRNA in the esophagus, but the amounts are low and may not be sufficient to account for NBzMA activation in this tissue.

Cytochrome P450

Since 1993, three new cytochrome P450 X-ray structures have been determined, giving a total of four known structures. Two of the new structures are in the substrate-free form and one is substrate-bound. These new structures, together with a wealth of mutagenesis studies on various P450s, have provided considerable information on what structural features control substrate specificity in P450s. In addition, some important insights into the catalytic mechanism have been made.

Cytochrome P450 2A of nasal epithelium: regulation and role in carcinogen metabolism

In this study, we found that rat nasal coumarin-7-hydroxylase (COH) activity was two orders of magnitude higher than rat hepatic COH activity and could be induced by adding coumarin to the rats' drinking water. In western blot analysis, an anti-cytochrome P450 (Cyp) 2a-5 (mouse liver COH) antibody recognized a sharp band in the microsomal fraction of rat nasal epithelium but not of the liver; the band comigrated with Cyp2a-5. The intensity of the band was increased by the coumarin treatment. Similarly, in northern blot analysis, a cDNA probe specific for Cyp2a-5 recognized an mRNA in the nasal epithelium having the same size as mouse liver Cyp2a-5 mRNA; however, no hybridizable mRNA was recognized in liver preparations. Unlike the protein level, the level of the mRNA was not increased by coumarin. When northern blot analyses were performed with two oligoprobes specific for rat lung CYP2A3, an mRNA of similar size to Cyp2a-5 mRNA was recognized. In immunoinhibition analysis, anti-Cyp2a-5 antibody inhibited rat nasal COH activity and aflatoxin B1 (AFB1) metabolism completely. It inhibited N-nitrosodiethylamine (NDEA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) metabolism by 80-90%. In contrast, the hepatic metabolism of the four compounds was not affected by the antibody. When coumarin instead of anti-Cyp2a-5 antibody was used, a strong but variable inhibition of the nasal metabolism of AFB1, NDEA, and NNK was seen. The results suggest that an enzyme or enzymes similar to mouse liver Cyp2a-5, one of which may be CYP2A3, is expressed at high levels in rat nasal epithelium but not in the liver and that its expression is increased by coumarin, an odorant and a substrate of Cyp2a-5. The increase probably occurs by protein stabilization or stimulation of translation. The results also show that the enzyme has a key role in the nasal metabolism of three well-known carcinogens, AFB1, NDEA, and NNK and may therefore be an important contributing factor in nasal carcinogenesis.