AMPK regulates phagophore-to-autophagosome maturation

Autophagy is an important metabolic pathway that can non-selectively recycle cellular material or lead to targeted degradation of protein aggregates or damaged organelles. Autophagosome formation starts with autophagy factors accumulating on lipid vesicles containing ATG9. These phagophores attach to donor membranes, expand via ATG2-mediated lipid transfer, capture cargo, and mature into autophagosomes, ultimately fusing with lysosomes for their degradation. Autophagy can be activated by nutrient stress, for example, by a reduction in the cellular levels of amino acids. In contrast, how autophagy is regulated by low cellular ATP levels via the AMP-activated protein kinase (AMPK), an important therapeutic target, is less clear. Using live-cell imaging and an automated image analysis pipeline, we systematically dissect how nutrient starvation regulates autophagosome biogenesis. We demonstrate that glucose starvation downregulates autophagosome maturation by AMPK-mediated inhibition of phagophore tethering to donor membrane. Our results clarify AMPKs regulatory role in autophagy and highlight its potential as a therapeutic target to reduce autophagy.

FooDOxS: a database of oxidized sterols content in foods

Dietary oxidized sterols (DOxS) are cholesterol-like molecules known to exert pro-inflammatory, pro-oxidant, and pro-apoptotic effects, among others. We present the FooDOxS database, a comprehensive compilation of DOxS content in over 1680 food items from 120 publications across 25 countries, augmented by data generated by our group. This database reports DOxS content in foods classified under the NOVA and What We Eat in America (WWEIA) systems, allowing a comprehensive and statistically robust summary of DOxS content in foods. Notably, we evaluated the efficacy of using NOVA and WWEIA classifications in capturing DOxS variations across food categories. Our findings provide insights into the strengths and limitations of these classification systems, enhancing their utility for assessing dietary components. This research contributes to the understanding of DOxS in food processing and suggests refinements for classification systems, holding promise for improved food safety and public health assessments.

Tracking the transition from an ATG9A vesicle to an autophagosome

ABBREVIATIONS

ATG: autophagy-related proteins; ULK1/2: Unc-51-Like activating Kinases; PI3Ks: Phosphoinositide 3-Kinases; ATG2A: autophagy-related protein 2A; ATG5: autophagy-related protein 5; ATG16: autophagy-related protein 16; ATG8: autophagy-related protein 8; U2OS: human bone osteosarcoma epithelial cell; LC3B: microtubule-associated protein 1A/1B Light Chain 3B; GABARAPL1: GABA type A Receptor-Associated Protein Like 1; ATG9A: autophagy-related protein 9A; ATG13: autophagy-related protein 13; SQSTM1: Sequestosome-1/p62; WIPI2: WD repeat domain, Phosphoinositide Interacting 2; PI3P: Phosphoinositide-3-phosphate.

Oxidative Status of Ultra-Processed Foods in the Western Diet

Ultra-processed foods (UPFs) have gained substantial attention in the scientific community due to their surging consumption and potential health repercussions. In addition to their well-established poor nutritional profile, UPFs have been implicated in containing various dietary oxidized sterols (DOxSs). These DOxSs are associated with a spectrum of chronic diseases, including cardiometabolic conditions, cancer, diabetes, Parkinson's, and Alzheimer's disease. In this study, we present a comprehensive database documenting the presence of DOxSs and other dietary metabolites in >60 UPFs commonly consumed as part of the Western diet. Significant differences were found in DOxS and phytosterol content between ready-to-eat (RTE) and fast foods (FFs). Biomarker analysis revealed that DOxS accumulation, particularly 25-OH and triol, can potentially discriminate between RTEs and FFs. This work underscores the potential utility of dietary biomarkers in early disease detection and prevention. However, an essential next step is conducting exposure assessments to better comprehend the levels of DOxS exposure and their association with chronic diseases.

Cholesterol crystals induce mechanical trauma, inflammation, and neo-vascularization in solid cancers as in atherosclerosis

Background and aims

Cancer and atherosclerosis share common risk factors and inflammatory pathways that promote their proliferation via vascular endothelial growth factor (VEGF). Because CCs cause mechanical injury and inflammation in atherosclerosis, we investigated their presence in solid cancers and their activation of IL-1β, VEGF, CD44, and Ubiquityl-Histone H2B (Ub-H2B), that promote cancer growth.

Methods

Tumor specimens from eleven different types of human cancers and atherosclerotic plaques were assessed for CCs, free cholesterol content and IL1-β by microscopy, immunohistochemistry, and biochemical analysis. Breast and colon cancer cell lines were cultured with and without CCs to select for expression of VEGF, CD44, and Ub-H2B. Western blot and immunofluorescence were performed on cells to assess the effect of CCs on signaling pathways.

Results

Cancers displayed higher CC content (+2.29 ± 0.74 vs +1.46 ± 0.84, p < 0.0001), distribution (5.06 ± 3.13 vs 2.86 ± 2.18, p < 0.001) and free cholesterol (3.63 ± 4.02 vs 1.52 ± 0.56 μg/mg, p < 0.01) than cancer free marginal tissues and similarly for atherosclerotic plaques and margins (+2.31 ± 0.51 vs +1.44 ± 0.79, p < 0.02; 14.0 ± 5.74 vs 8.14 ± 5.52, p < 0.03; 0.19 ± 0.14 vs 0.09 ± 0.04 μg/mg, p < 0.02) respectively. Cancers displayed significantly increased expression of IL1-β compared to marginal tissues. CCs treated cancer cells had increased expression of VEGF, CD44, and Ub-H2B compared to control. By microscopy, CCs were found perforating cancer tumors similar to plaque rupture.

Conclusions

These findings suggest that CCs can induce trauma and activate cytokines that enhance cancer growth as in atherosclerosis.

AMPK Regulates Phagophore-to-Autophagosome Maturation

Autophagy is an important metabolic pathway that can non-selectively recycle cellular material or lead to targeted degradation of protein aggregates or damaged organelles. Autophagosome formation starts with autophagy factors accumulating on lipid vesicles containing ATG9. These phagophores attach to donor membranes, expand via ATG2-mediated lipid transfer, capture cargo, and mature into autophagosomes, ultimately fusing with lysosomes for their degradation. Autophagy can be activated by nutrient stress, for example by a reduction in the cellular levels of amino acids. In contrast, how autophagy is regulated by low cellular ATP levels via the AMP-activated protein kinase (AMPK), an important therapeutic target, is less clear. Using live-cell imaging and an automated image analysis pipeline, we systematically dissect how nutrient starvation regulates autophagosome biogenesis. We demonstrate that glucose starvation downregulates autophagosome maturation by AMPK mediated inhibition of phagophores tethering to donor membranes. Our results clarify AMPK's regulatory role in autophagy and highlight its potential as a therapeutic target to reduce autophagy.

Quantitative analysis of autophagy reveals the role of ATG9 and ATG2 in autophagosome formation

Autophagy is a catabolic pathway required for the recycling of cytoplasmic materials. To define the mechanisms underlying autophagy it is critical to quantitatively characterize the dynamic behavior of autophagy factors in living cells. Using a panel of cell lines expressing HaloTagged autophagy factors from their endogenous loci, we analyzed the abundance, single-molecule dynamics, and autophagosome association kinetics of autophagy proteins involved in autophagosome biogenesis. We demonstrate that autophagosome formation is inefficient and ATG2-mediated tethering to donor membranes is a key commitment step in autophagosome formation. Furthermore, our observations support the model that phagophores are initiated by the accumulation of autophagy factors on mobile ATG9 vesicles, and that the ULK1 complex and PI3-kinase form a positive feedback loop required for autophagosome formation. Finally, we demonstrate that the duration of autophagosome biogenesis is ∼110 s. In total, our work provides quantitative insight into autophagosome biogenesis and establishes an experimental framework to analyze autophagy in human cells.

Dietary exposure assessment of infant formula and baby foods' oxidized lipids in the US population

Baby Foods (BFs) and Infant formulas (IFs) are the main sources of nutrition for an infant throughout the 1st year of life. Various enriched products are commercially available for parents seeking to fulfill their baby's nutritional needs. Consequently, different bioactive lipids are present in BFs and IFs, including dietary oxysterols (DOxS), whose known toxicity has been associated with mutagenicity, cancer, and other chronic diseases. In this work, we performed an exposure assessment of 25 bioactive lipids on IFs (n = 30) and BFs (n = 13) commercially available in the US. To determine dietary exposure, we used EPA's SHEDS-HT probabilistic model. Even though β-Sitosterol was the most exposed bioactive lipid with 75,410 μg/day, cholesterol was the most absorbed compound during the entire first year (19.3 mg/day). Additionally, we found 7α-hydroxycholesterol (7α-OH) as a potential DOxS biomarker of the BFs manufacturing process. This is the first time an infant's exposure assessment (including DOxS) after BFs and IFs consumption is performed, enabling much-needed information regarding these hazardous compounds and their potential effects on infants' health.

Evaluation of the nutritional quality of ultra-processed foods (ready to eat + fast food): Fatty acids, sugar, and sodium

The average American consumes more than 50% of their total dietary energy from ultra-processed foods (UPFs). From a nutritional standpoint, as UPFs intake increases, fiber, vitamin, and mineral intake decrease. High consumption of UPFs, mainly from fast foods (FF) and ready-to-eat (RTE) food items, emerges as a critical public health concern linking nutritional quality and food safety. In the present work, a systematic database of the fatty acid composition of the most consumed UPFs in the Midwest is reported. Saturated and monounsaturated fatty acids were predominant in RTE (42.5%) and FF (43.2%), respectively. In addition, the fatty acid profile in UPFs is reported according to six food categories: meat and poultry, eggs and derivatives, dairy products, seafood, baby foods, and others. Meat and poultry, and dairy products were the dominant food categories among UPFs. Meanwhile, polyunsaturated fatty acids were abundant in the eggs and seafood groups UPFs (61.8% and 46.4%, respectively) regardless of the food group. Furthermore, no significant differences were found in sugar content in UPFs. Caloric content was positively correlated with sodium (ρ = 0.748) and price (ρ = 0.534). The significance of this study relies on providing new quantitative data on the fat, sodium, and sugar contents of the most consumed UPFs in the Midwestern area of the United States. This information suggests paying more attention to these nutritional attributes, aiming to reduce their incorporation in UPF preparations. Additionally, more quantitative data are needed regarding other nutritional parameters such as protein and lipid degradation in UPFs. PRACTICAL APPLICATION: This study provides a profile of the fatty acid composition of the most consumed UPFs in the Midwestern region of the United States, as well as correlations with fat, sodium, and sugar contents in UPFs. The information offered a new perspective on the nutrition quality of UPFs, suggesting the reduction of the incorporation of these attributes in UPFs. Additionally, it will help define priority interventions for more advanced precision nutrition, especially for vulnerable populations, for example, children and older people. The overall decrease in added sugar and sodium and the service size in UPFs will significantly improve the nutritional quality of the Western diet.

Lipid profiling and dietary assessment of infant formulas reveal high intakes of major cholesterol oxidative product (7-ketocholesterol)

Approximately two-thirds of US infants receive infant formula (IF) as a primary or sole nutritional source during the first six months of life. IF is available in a variety of commercial presentations; from a manufacturing standpoint, they can be categorized as powder- (PIF) or liquid- (LIF) based formulations. Thirty commercial IFs were analyzed in their oxidative and non-oxidative lipid profiles. We identified 7-ketocholesterol - a major end-product of cholesterol oxidation - as a potential biomarker of IF manufacturing. The statistical analysis allowed a re-classification of IF based on their metabolomic fingerprint, resulting in three groups assigned with low-to-high oxidative status. Finally, we modeled the dietary intake of cholesterol, sterols, and 7-ketocholesterol in the first year of life. The database provided in this study will be instrumental for scientists interested in infant nutrition, to establish bases for epidemiological studies aimed to find connections between nutrition and diet-associated diseases, such as sitosterolemia.

Chemical characterization and enological potential of less frequent red grape Uruguayan varieties by study secondary metabolites

Study of secondary grape metabolites, such as polyphenol compounds (anthocyanins, flavonols, flavan-3-ols), volatile compounds present in grapes in both free and glycoside forms (monoterpenols, norisoprenoids, benzenoids), allowed us to characterize grape varieties and to suggest the best winemaking practices to maximize their enological potential. The Vitis viníferas studied in the present work are less frequent red grape varieties cultivated in small vineyards in southern Uruguay (Montevideo and Canelones Provinces): Ancellota, Aspiran Bouschet (syn. Lacryma Christi), Marselan (Grenache × Cabernet Sauvignon), Arinarnoa (Tannat × Cabernet Sauvignon), Egiodola (Abouriou × Tinta Negra Mole) and Caladoc (Malbec × Grenache).

Lipid-exchange in nanodiscs discloses membrane boundaries of cytochrome-P450 reductase

Lipids are critical for the function of membrane proteins. NADPH-cytochrome-P450-reductase, the sole electron transferase for microsomal oxygenases, possesses a conformational dynamics entwined with its topology. Here, we use peptide-nanodiscs to unveil cytochrome-P450-reductase's lipid boundaries, demonstrating a protein-driven enrichment of ethanolamine lipids (by 25%) which ameliorates by 3-fold CPR's electron-transfer ability.

Non-targeted glycosidic profiling of international wines using neutral loss-high resolution mass spectrometry

Many metabolites naturally occur as glycosides, since sugar moieties can be crucial for their biological activity and increase their water solubility. In the plant kingdom they may occur as glycosides or sugar esters, depending on precursor chemical structure, and in wine they have traditionally attracted attention due to their organoleptic properties, such as astringency and bitterness, and because they affect the colour and aroma of wines. A new approach directed at detailed description of glycosides in a large selection of monovarietal wines (8 samples each of Pinot Blanc, Muller Thurgau, Riesling, Traminer, Merlot, Pinot Noir and Cabernet Sauvignon) was developed by combining high performance liquid chromatography with high resolution tandem mass spectrometry. Analytical separation was performed on an Accucore™ Polar Premium LC column, while mass analysis was performed in negative ion mode with an non-targeted screening approach, using a Full MS/AIF/NL dd-MS² experiment at a resolving power of 140,000 FWHM. Over 280 glycoside-like compounds were detected, of which 133 (including low-molecular weight phenols, flavonoids and monoterpenols) were tentatively identified in the form of pentose (6), deoxyhexose (17), hexose (73), hexose-pentose (16), hexose-deoxyhexose (7), dihexose (5) and hexose ester (9) derivatives. It was not possible to univocally define the corresponding chemical structure for the remaining 149 glycosides. Non-parametric statistical analysis showed it was possible to well characterise the glycosylated profile of all red and Traminer wines, while the identified glycosides were almost entirely lacking in Pinot Blanc, Riesling and Muller Thurgau wines. Also Tukey's Honestly Significant Difference test (p < 0.05) and Principal Component Analysis confirmed that it was possible to almost entirely distinguish the selected red wines from each other according to their glycosylated profile.

Membrane environment drives cytochrome P450's spin transition and its interaction with cytochrome b5

Heme's spin-multiplicity is key in determining the enzymatic function of cytochrome P450 (cytP450). The origin of the low-spin state in ferric P450 is still under debate. Here, we report the first experimental demonstration of P450's membrane interaction altering its spin equilibrium which is accompanied by a stronger affinity for cytochrome b₅. These results highlight the importance of lipid membrane for the function of P450.

Cytochrome-P450-Induced Ordering of Microsomal Membranes Modulates Affinity for Drugs

Although membrane environment is known to boost drug metabolism by mammalian cytochrome P450s, the factors that stabilize the structural folding and enhance protein function are unclear. In this study, we use peptide-based lipid nanodiscs to "trap" the lipid boundaries of microsomal cytochrome P450 2B4. We report the first evidence that CYP2B4 is able to induce the formation of raft domains in a biomimetic compound of the endoplasmic reticulum. NMR experiments were used to identify and quantitatively determine the lipids present in nanodiscs. A combination of biophysical experiments and molecular dynamics simulations revealed a sphingomyelin binding region in CYP2B4. The protein-induced lipid raft formation increased the thermal stability of P450 and dramatically altered ligand binding kinetics of the hydrophilic ligand BHT. These results unveil membrane/protein dynamics that contribute to the delicate mechanism of redox catalysis in lipid membrane.

Dissociation Constants of Cytochrome P450 2C9/Cytochrome P450 Reductase Complexes in a Lipid Bilayer Membrane Depend on NADPH: A Single-Protein Tracking Study

Cytochrome P450-reductase (CPR) is a versatile NADPH-dependent electron donor located in the cytoplasmic side of the endoplasmic reticulum. It is an electron transferase that is able to deliver electrons to a variety of membrane-bound oxidative partners, including the drug-metabolizing enzymes of the cytochrome P450s (P450). CPR is also stoichiometrically limited compared to its oxidative counterparts, and hypotheses have arisen about possible models that can overcome the stoichiometric imbalance, including quaternary organization of P450 and diffusion-limited models. Described here are results from a single-protein tracking study of fluorescently labeled CPR and cytochrome P450 2C9 (CYP2C9) molecules in which stochastic analysis was used to determine the dissociation constants of CPR/CYP2C9 complexes in a lipid bilayer membrane for the first time. Single-protein trajectories demonstrate the transient nature of these CPR-CYP2C9 interactions, and the measured K_d values are highly dependent on the redox state of CPR. It is shown that CPR^ox/CYP2C9 complexes have a much higher dissociation constant than CPR^2-/CYP2C9 or CPR^4-/CYP2C9 complexes, and a model is presented to account for these results. An Arrhenius analysis of diffusion constants was also carried out, demonstrating that the reduced forms of CPR and CYP2C9 interact differently with the biomimetic ER and may, in addition to protein conformational changes, contribute to the observed NADPH-dependent shift in K_d. Finally, it is also shown that the CPR^ox/CYP2C9 affinity depends on the nature of the ligand, being higher when a substrate is bound, compared to an inhibitor.

Kinetic and Structural Characterization of the Effects of Membrane on the Complex of Cytochrome b 5 and Cytochrome c

Cytochrome b₅ (cytb₅) is a membrane protein vital for the regulation of cytochrome P450 (cytP450) metabolism and is capable of electron transfer to many redox partners. Here, using cyt c as a surrogate for cytP450, we report the effect of membrane on the interaction between full-length cytb₅ and cyt c for the first time. As shown through stopped-flow kinetic experiments, electron transfer capable cytb₅ - cyt c complexes were formed in the presence of bicelles and nanodiscs. Experimentally measured NMR parameters were used to map the cytb₅-cyt c binding interface. Our experimental results identify differences in the binding epitope of cytb₅ in the presence and absence of membrane. Notably, in the presence of membrane, cytb₅ only engaged cyt c at its lower and upper clefts while the membrane-free cytb₅ also uses a distal region. Using restraints generated from both cytb₅ and cyt c, a complex structure was generated and a potential electron transfer pathway was identified. These results demonstrate the importance of studying protein-protein complex formation in membrane mimetic systems. Our results also demonstrate the successful preparation of novel peptide-based lipid nanodiscs, which are detergent-free and possesses size flexibility, and their use for NMR structural studies of membrane proteins.

The catalytic function of cytochrome P450 is entwined with its membrane-bound nature

Cytochrome P450, a family of monooxygenase enzymes, is organized as a catalytic metabolon, which requires enzymatic partners as well as environmental factors that tune its complex dynamic. P450 and its reducing counterparts—cytochrome P450-reductase and cytochrome b ₅—are membrane-bound proteins located in the cytosolic side of the endoplasmic reticulum. They are believed to dynamically associate to form functional complexes. Increasing experimental evidence signifies the role(s) played by both protein-protein and protein-lipid interactions in P450 catalytic function and efficiency. However, the biophysical challenges posed by their membrane-bound nature have severely limited high-resolution understanding of the molecular interfaces of these interactions. In this article, we provide an overview of the current knowledge on cytochrome P450, highlighting the environmental factors that are entwined with its metabolic function. Recent advances in structural biophysics are also discussed, setting up the bases for a new paradigm in the study of this important class of membrane-bound enzymes.

Single-Protein Tracking Reveals That NADPH Mediates the Insertion of Cytochrome P450 Reductase into a Biomimetic of the Endoplasmic Reticulum

Cytochrome P450 reductase (CPR) is the redox partner for most human cytochrome P450 enzymes. It is also believed that CPR is an integral membrane protein exclusively. Herein, we report that, contrary to this belief, CPR can exist as a peripheral membrane protein in the absence of NADPH and will transition to an integral membrane protein in the presence of stoichiometric amounts of NADPH or greater. All experiments were performed in a solid-supported cushioned lipid bilayer that closely matched the chemical composition of the human endoplasmic reticulum and served as an ER biomimetic. The phase characteristics and fluidity of the ER biomimetic was characterized with fluorescence micrographs and temperature-dependent fluorescence recovery after photobleaching. The interactions of CPR with the ER biomimetic were directly observed by tracking single CPR molecules using time-lapse single-molecule fluorescence imaging and subsequent analysis of tracks. These studies revealed dramatic changes in diffusion coefficient and the degree of partitioning of CPR as a function of NADPH concentration.

Characterization of Class III Peroxidases from Switchgrass

Class III peroxidases (CIIIPRX) catalyze the oxidation of monolignols, generate radicals, and ultimately lead to the formation of lignin. In general, CIIIPRX genes encode a large number of isozymes with ranges of in vitro substrate specificities. In order to elucidate the mode of substrate specificity of these enzymes, we characterized one of the CIIIPRXs (PviPRX9) from switchgrass (Panicum virgatum), a strategic plant for second-generation biofuels. The crystal structure, kinetic experiments, molecular docking, as well as expression patterns of PviPRX9 across multiple tissues and treatments, along with its levels of coexpression with the majority of genes in the monolignol biosynthesis pathway, revealed the function of PviPRX9 in lignification. Significantly, our study suggested that PviPRX9 has the ability to oxidize a broad range of phenylpropanoids with rather similar efficiencies, which reflects its role in the fortification of cell walls during normal growth and root development and in response to insect feeding. Based on the observed interactions of phenylpropanoids in the active site and analysis of kinetics, a catalytic mechanism involving two water molecules and residues histidine-42, arginine-38, and serine-71 was proposed. In addition, proline-138 and gluntamine-140 at the 137P-X-P-X140 motif, leucine-66, proline-67, and asparagine-176 may account for the broad substrate specificity of PviPRX9. Taken together, these observations shed new light on the function and catalysis of PviPRX9 and potentially benefit efforts to improve biomass conservation properties in bioenergy and forage crops.

Cholesterol photo-oxidation: A chemical reaction network for kinetic modeling

In this work we studied the effect of polyunsaturated fatty acids (PUFAs) methyl esters on cholesterol photo-induced oxidation. The oxidative routes were modeled with a chemical reaction network (CRN), which represents the first application of CRN to the oxidative degradation of a food-related lipid matrix. Docosahexaenoic acid (DHA, T-I), eicosapentaenoic acid (EPA, T-II) and a mixture of both (T-III) were added to cholesterol using hematoporphyrin as sensitizer, and were exposed to a fluorescent lamp for 48h. High amounts of Type I cholesterol oxidation products (COPs) were recovered (epimers 7α- and 7β-OH, 7-keto and 25-OH), as well as 5β,6β-epoxy. Fitting the experimental data with the CRN allowed characterizing the associated kinetics. DHA and EPA exerted different effects on the oxidative process. DHA showed a protective effect to 7-hydroxy derivatives, whereas EPA enhanced side-chain oxidation and 7β-OH kinetic rates. The mixture of PUFAs increased the kinetic rates several fold, particularly for 25-OH. With respect to the control, the formation of β-epoxy was reduced, suggesting potential inhibition in the presence of PUFAs.

Mechanism-Based Inhibition of CYP3A4 by Podophyllotoxin: Aging of an Intermediate Is Important for in Vitro/in Vivo Correlations

An in vitro observation of time-dependent inhibition (TDI) of metabolic enzymes often results in removing a potential drug from the drug pipeline. However, the accepted method for predicting TDIs of the important drug metabolizing cytochrome P450 enzymes often overestimates the drug interaction potential. Better models that take into account the complexities of the cytochrome P450 enzyme system will lead to better predictions. Herein we report the use of our previously described models for complex kinetics of podophyllotoxin. Spectral characterization of the kinetics indicates that an intermediate MI complex is formed, which slowly progresses to an essentially irreversible MI complex. The intermediate MI complex can release free enzyme during the time course of a typical 30 min TDI experiment. This slow rate of MI complex conversion results in an overprediction of the kinact value if this process is not included in the analysis of the activity versus time profile. In vitro kinetic experiments in rat liver microsomes predicted a lack of drug interaction between podophyllotoxin and midazolam. In vivo rat pharmacokinetic studies confirmed this lack of drug interaction.

The Role of Protein-Protein and Protein-Membrane Interactions on P450 Function

This symposium summary, sponsored by the ASPET, was held at Experimental Biology 2015 on March 29, 2015, in Boston, Massachusetts. The symposium focused on: 1) the interactions of cytochrome P450s (P450s) with their redox partners; and 2) the role of the lipid membrane in their orientation and stabilization. Two presentations discussed the interactions of P450s with NADPH-P450 reductase (CPR) and cytochrome b5. First, solution nuclear magnetic resonance was used to compare the protein interactions that facilitated either the hydroxylase or lyase activities of CYP17A1. The lyase interaction was stimulated by the presence of b5 and 17α-hydroxypregnenolone, whereas the hydroxylase reaction was predominant in the absence of b5. The role of b5 was also shown in vivo by selective hepatic knockout of b5 from mice expressing CYP3A4 and CYP2D6; the lack of b5 caused a decrease in the clearance of several substrates. The role of the membrane on P450 orientation was examined using computational methods, showing that the proximal region of the P450 molecule faced the aqueous phase. The distal region, containing the substrate-access channel, was associated with the membrane. The interaction of NADPH-P450 reductase (CPR) with the membrane was also described, showing the ability of CPR to "helicopter" above the membrane. Finally, the endoplasmic reticulum (ER) was shown to be heterogeneous, having ordered membrane regions containing cholesterol and more disordered regions. Interestingly, two closely related P450s, CYP1A1 and CYP1A2, resided in different regions of the ER. The structural characteristics of their localization were examined. These studies emphasize the importance of P450 protein organization to their function.

Substrate Dependent Native Luminescence from Cytochromes P450 3A4, 2C9, and P450cam

Metalloporphyrin containing proteins, such as cytochrome P450, play a key role in biological systems. The spectroscopic properties of metalloporphyrins have been a subject of intense interest and intense debate for over 50 years. Iron-porphyrins are usually believed to be nonfluorescent. Herein we report that, contrary to this belief, cytochrome P450 heme groups luminesce with enough intensity to be of use in the characterization of these enzymes. To confirm that the emission is from the heme, we destroyed the heme by titration with cumene hydroperoxide and measured the changes in emission upon titration with compounds known to bind to the distal face of the heme in two human cytochrome P450 enzymes, known as CYP3A4 and CYP2C9. The titration curves gave spectral dissociation constants that were not significantly different from those reported using the Soret UV/vis absorbance changes. We have tentatively assigned the broad luminescence at ∼500 nm to a (1)ππ* → gs fluorescence and the structured luminescence above 600 nm to a (3)ππ* → gs phosphorescence. These assignments are not associated with the Q-band, and are in violation of Kasha's rule. To illustrate the utility of the emission, we measured spectral dissociation constants for testosterone binding to P450 3A4 in bilayers formed on glass coverslips, a measurement that would be very difficult to make using absorption spectroscopy. Complementary experiments were carried out with water-soluble P450cam.

Kinetics of 25-hydroperoxycholesterol formation during photo-oxidation of crystalline cholesterol

BACKGROUND

25-Hydroxycholesterol (25-OH), a side-chain product of cholesterol oxidation, has emerged as one of the important issues in food chemistry and biochemistry, because of its involvement in several human pathologies. This oxysterol is derived from both enzymatic and non-enzymatic pathways. However, the latter mechanism has been scarcely studied in either food or model systems. In this work, a kinetic model was developed to evaluate the formation of 25-OH and its precursor 25-hydroperoxycholesterol (25-OOH) during photo-oxidation of cholesterol for 28 days under fluorescent light. 25-OOH was estimated by an indirect method, using thin-layer chromatography coupled with gas chromatography-mass spectrometry.

RESULTS

Peroxide value (POV) and cholesterol oxidation products (COPs) were determined. POV showed a hyperbolic behavior, typical of a crystalline system in which the availability of cholesterol is the limiting factor. Further reactions of hydroperoxides were followed; in particular, after photo-oxidation, 25-OOH (0.55 mg g(-1) ) and 25-OH (0.08 mg g(-1) ) were found in cholesterol, as well as seven other oxysterols, including 7-hydroxy and 5,6-epoxy derivatives. The application of kinetic models to the data showed good correlation with theoretical values, allowing derivation of the kinetic parameters for each oxidation route.

CONCLUSIONS

The results of this work confirm that cholesterol in the crystalline state involves different oxidation patterns as compared to cholesterol in solution. Moreover, the numerical fit proved that hydroperoxidation is the rate-limiting step in 25-OH formation.

In vitro oxidative metabolism of 6-mercaptopurine in human liver: insights into the role of the molybdoflavoenzymes aldehyde oxidase, xanthine oxidase, and xanthine dehydrogenase

Anticancer agent 6-mercaptopurine (6MP) has been in use since 1953 for the treatment of childhood acute lymphoblastic leukemia (ALL) and inflammatory bowel disease. Despite being available for 60 years, several aspects of 6MP drug metabolism and pharmacokinetics in humans are unknown. Molybdoflavoenzymes such as aldehyde oxidase (AO) and xanthine oxidase (XO) have previously been implicated in the metabolism of this drug. In this study, we investigated the in vitro metabolism of 6MP to 6-thiouric acid (6TUA) in pooled human liver cytosol. We discovered that 6MP is metabolized to 6TUA through sequential metabolism via the 6-thioxanthine (6TX) intermediate. The role of human AO and XO in the metabolism of 6MP was established using the specific inhibitors raloxifene and febuxostat. Both AO and XO were involved in the metabolism of the 6TX intermediate, whereas only XO was responsible for the conversion of 6TX to 6TUA. These findings were further confirmed using purified human AO and Escherichia coli lysate containing expressed recombinant human XO. Xanthine dehydrogenase (XDH), which belongs to the family of xanthine oxidoreductases and preferentially reduces nicotinamide adenine dinucleotide (NAD(+)), was shown to contribute to the overall production of the 6TX intermediate as well as the final product 6TUA in the presence of NAD(+) in human liver cytosol. In conclusion, we present evidence that three enzymes, AO, XO, and XDH, contribute to the production of 6TX intermediate, whereas only XO and XDH are involved in the conversion of 6TX to 6TUA in pooled HLC.