Environmental biotransformation mechanisms by flavin-dependent monooxygenase: A computational study.

The enzyme-catalyzed metabolic biotransformation of xenobiotics plays a significant role in toxicology evolution and subsequently environmental health risk assessment. Recent studies noted that the phase I human flavin-dependent monooxygenase (e.g., FMO3) can catalyze xenobiotics into more toxic metabolites. However, details of the metabolic mechanisms are insufficient. To fill the mechanism in the gaps, the systemic density functional theory calculations were performed to elucidate diverse FMO-catalyzed oxidation reactions toward environmental pollutants, including denitrification (e.g., nitrophenol), N-oxidation (e.g., nicotine), desulfurization (e.g., fonofos), and dehalogenation (e.g., pentachlorophenol). Similar to the active center compound 0 of cytochrome P450, FMO mainly catalyzed reactions with the structure of the tricyclic isoalloxazine C-4a-hydroperoxide (FADHOOH). As will be shown, FMO-catalyzed pathways are more favorable with a concerted than stepwise mechanism; Deprotonation is necessary to initiate the oxidation reactions for phenolic substrates; The regioselectivity of nicotine by FMO prefers the N-oxidation other than N-demethylation pathway; Formation of the P-S-O triangle ring is the key step for desulfurization of fonofos by FMO. We envision that these fundamental mechanisms catalyzed by FMO with a computational method can be extended to other xenobiotics of similar structures, which may aid the high-throughput screening and provide theoretical predictions in the future.

Biotransformation Mechanism of Pesticides by Cytochrome P450: A DFT Study on Dieldrin.

Pesticide biotransformation, especially by cytochrome P450 enzymes (CYPs), may produce metabolites with substantially altered toxicological and physicochemical profiles, which has drawn great attention as a basis for environmental risk assessment. CYPs are active in the metabolism of various reactions of pesticides, and there are potentially different short-lived oxidant species in CYPs (Compound I vs Compound 0), which make elucidating their biotransformation mechanism challenging. To facilitate this task, we performed density functional theory (DFT) calculations to explore the puzzling bifurcation pathways of dieldrin by CYPs. The results show that the two-oxidant mechanism does not work, while the bifurcation pathways are within the mechanistic framework of a two-state reactivity of Compound I. Specifically, 9-hydroxy-dieldrin as a hydroxylation product is formed via H-abstraction and essentially barrierless C-9 alkyl radical rebound in the doublet state; while 3-ketone-dieldrin as a dechlorination product is formed via H-abstraction, C-9 alkyl radical cyclization, and C-3 cyclized radical rebound in the quartet state followed by HCl elimination, originating from a significant barrier for C-9 alkyl radical rebound in the quartet state to provide this radical sufficient lifetime for cyclization. Thus, the ratio [dechlorination]/[hydroxylation] can be estimated as 1:35, consistent with the experimental findings. We envision that application of computational chemistry has a great potential in revealing the complex biotransformation mechanisms of pesticides.

Changes in Lipid Indices in HIV+ Cases on HAART.

We assess long-term changes in lipid levels in human immunodeficiency disease- (HIV-) infected patients undergoing highly active antiretroviral treatment (HAART) and their association with diabetes mellitus (DM) and thyroid dysfunction. We observed changes in the levels of total cholesterol (TC) and total triglyceride (TG) of 63 HIV-infected patients in the 6 years from starting HAART and analyzed correlations between relevant parameters. TC levels of patients with normal baseline TC levels as well as those diagnosed with DM or impaired fasting glucose (IFG) increased significantly (P < 0.05) as did the TG levels of patients with normal baseline TG levels (P < 0.05). TC levels of patients with hypercholesterolemia in the year HAART was initiated were significantly higher than those of patients with normal baseline TC levels (P < 0.05) for all 6 years. TC levels of patients diagnosed with DM were significantly higher than those with euglycemia (P < 0.05) 2 and 4 years after HAART commencement. Levels of TC, high-density lipoprotein-cholesterol (HDL-C), and low-density lipoprotein-cholesterol (LDL-C) were correlated negatively with viral load, whereas levels of TC and very-low-density lipoprotein-cholesterol (VLDL-C) were correlated positively with CD4+ cell counts before HAART commencement. Linear mixed-effect model demonstrated disturbance of glucose metabolism and HAART containing nevirapine and CD4+ cell count were positively correlated with TC levels after HAART commencement. These findings suggest that there are changes in the lipid levels of patients undergoing HAART, with the potential risk of dyslipidemia.