Contributions of UDP-Glucuronosyltransferases to Human Hepatic and Intestinal Metabolism of Ticagrelor and Inhibition of UGTs and Cytochrome P450 Enzymes by Ticagrelor and its Glucuronidated Metabolite

Ticagrelor is the first reversibly binding, direct-acting, oral P2Y12 receptor inhibitor. The contribution of UDP-glucuronosyltransferases (UGTs) enzymes to the metabolism of ticagrelor to its glucuronide conjugation, ticagrelor-O-glucuronide, in human liver microsomes (HLM) and human intestinal microsomes (HIM), was well characterized in the current study. The inhibition potential of human major UGTs by ticagrelor and ticagrelor-O-glucuronide was explored. The inhibitory effects of ticagrelor-O-glucuronide on cytochrome P450s (CYPs) enzymes were investigated as well. Ticagrelor glucuronidation exhibits substrate inhibition kinetics in both HLM and HIM with apparent Km values of 5.65 and 2.52 μM, Vmax values of 8.03 and 0.90 pmol min−1·mg protein−1, Ksi values of 1,343.0 and 292.9 respectively. The in vitro intrinsic clearances (Vmax/Km) for ticagrelor glucuronidation by HLM and HIM were 1.42 and 0.36 μl min−1·mg protein−1, respectively. Study with recombinant human UGTs suggested that multiple UGT isoforms including UGT1A9, UGT1A7, UGT1A3, UGT1A4, UGT1A1, UGT2B7 and UGT1A8 are involved in the conversion of ticagrelor to ticagrelor-O-glucuronide with UGT1A9 showing highest catalytic activity. The results were further supported by the inhibition studies on ticagrelor glucuronidation with typical UGT inhibitors in pooled HLM and HIM. Little or no inhibition of UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9 and UGT2B7 by ticagrelor and ticagrelor-O-glucuronide was noted. Ticagrelor-O-glucuronide also exhibited limited inhibitory effects toward CYP2C8, CYP2D6 and CYP3A4. In contrast, ticagrelor-O-glucuronide weakly inhibited CYP2B6, CYP2C9 and CYP2C19 activity with apparent IC50 values of 45.0, 20.0 and 18.8 μM, respectively. The potential of ticagrelor-O-glucuronide to cause drug-drug interactions warrant further study.

Uncovering the cytochrome P450-catalyzed methylenedioxy bridge formation in streptovaricins biosynthesis

Streptovaricin C is a naphthalenic ansamycin antibiotic structurally similar to rifamycins with potential anti-MRSA bioactivities. However, the formation mechanism of the most fascinating and bioactivity-related methylenedioxy bridge (MDB) moiety in streptovaricins is unclear. Based on genetic and biochemical evidences, we herein clarify that the P450 enzyme StvP2 catalyzes the MDB formation in streptovaricins, with an atypical substrate inhibition kinetics. Furthermore, X-ray crystal structures in complex with substrate and structure-based mutagenesis reveal the intrinsic details of the enzymatic reaction. The mechanism of MDB formation is proposed to be an intramolecular nucleophilic substitution resulting from the hydroxylation by the heme core and the keto-enol tautomerization via a crucial catalytic triad (Asp89-His92-Arg72) in StvP2. In addition, in vitro reconstitution uncovers that C6-O-methylation and C4-O-acetylation of streptovaricins are necessary prerequisites for the MDB formation. This work provides insight for the MDB formation and adds evidence in support of the functional versatility of P450 enzymes.

Kinetic of 4-Chlorophenols Biodegradation by Arthrobacter Chlorophenolicus A6

Chlorophenols are listed as priority pollutants both by European community and US EPA. Biodegradation of pchlorophenol (4-CP) was investigated in batch shake flasks by Arthrobacter chlorophenolicus A6 at initial 4-CP concentrations between 25 to 350 mgl-1. The rate of 4-CP removal decreased with increasing initial 4-CP concentrations due to toxic effects on the microorganisms. The growth and biodegradation kinetic of the culture was evaluated. The batch growth profile of the A chlorophenolicus A6 followed substrate inhibition kinetics with the estimated biokinetic parameters of Ksi = 272 mgl-1, Ks = 65 mgl-1 for 4-CP respectively. High inhibition constant (KSI = 272 mg -l) with a KSI/KS ratio of 4.18 indicates superior 4-CP biodegradation potential of the A chlorophenolicus A6. The maximum rate of 4-CP degradation has been achieved at an optimum substrate concentration of Smax = (KSKSI)1/2 = (65×272)1/2 = 133 mgl−1.

Robust regulation of hepatic pericentral amination by glutamate dehydrogenase kinetics

α-Ketoglutarate substrate inhibition kinetics of hepatic glutamate dehydrogenase amination activity confers robust control over the ratio between glutamate and ammonium.

Study on Substrate Inhibition Kinetics in Anammox-UASB Reactor

The study on substrate inhibition kinetics in Anammox-UASB reactor was carried out treating low C/N wastewater. Haldane model was researched through non-linear fitting to represent the inhibition caused by nitrite. The results showed that Haldane model was suitable for fitting dynamics model. In Haldane model, rmax was 1.15 kg/( m3•d), KS=9.31 mg/L, KIH=476 mg/L. On condition that the concentration of nitrite was 66.57 mg/L, substrate conversion rate reached the highest value which was 0.90 kg/(m3•d) actually. And it was 78.1% of the highest value in theory.