- Effects of pH and temperature on the reaction of milk xanthine oxidase with 1-methylxanthine
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The reaction of milk Xanthine Oxidase (XO) with 1-methylxanthine has been investigated by steady state and stopped flow transient kinetic studies to understand the effect of a methyl group substituent on the purine ring. The pH dependence of the steady state kinetic parameter (Vmax/Km) shows a bell-shaped curve implying at least two ionisable groups are involved in the binding of XO with 1-methylxanthine, the higher pKa 7.7 corresponding to ionisation of the substrate and the lower one 6.2 to the enzyme (possibly Glu-1261 by analogy to Glu-869 of aldehyde oxidoreductase from Desulfovibrio gigas). The temperature dependence of the steady state and transient kinetic studies suggests the existence of at least one molecular intermediate during breakdown of the enzymesubstrate complex. The thermodynamic parameters of the microscopic rate constants were determined from the temperature dependence studies. The Royal Society of Chemistry 2000.
- Sau, Apurba Kurnar,Mondai, Madhu Sudan,Mitra, Samarcsh
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- Inhibition kinetics of theophylline metabolism by mexiletine and its metabolites
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To further characterize the mode of drug interaction between theophylline (TP) and mexiletine (ME), in vitro kinetic studies were carried out using rat liver microsomes and 9000 x g supernatant. The kinetic study revealed that the K, value and V(max)/K(m) ratio for the metabolic conversion of TP to 1,3-dimethyluric acid (1,3-DMU) were the second lowest and the highest, respectively, of four metabolic pathways. Thus, the rank of efficiency of the oxidative metabolism by microsomal cytochrome P-450 (P-450) isozymes was TP to 1,3-DMU > TP to 1-methylxanthine (1-MX) > TP to 3-MX > 1,3-DMU to 1-methyluric acid, suggesting that the isozyme metabolizing TP would have a higher affinity for the oxidation at the 8-position in TP molecules than at the 1- and 3-positions. Lineweaver-Burk plots showed that the conversion of TP to 3-MX and to 1,3-DIMU was inhibited competitively by ME and its metabolites, and that the pathway of TP to 1-MX was inhibited noncompetitively. In consideration of the K(i) values calculated, it seems probable that deamino-p-hydroxy ME (DApHME) might be the most potent inhibitor of the metabolic pathways of TP, and that the rank order of inhibition is approximately DApHME > p-hydroxy ME > deamino-hydroxymethyl ME > ME > hydroxymethyl ME, with some exceptions. The mechanism of the interaction between TP and ME is probably due to the metabolic antagonism in the liver, and TP, ME and their metabolites share' some of the same metabolic pathways, mediated by P-450 isozymes.
- Ogiso,Iwaki,Uno
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- Synthesis of 3-Alkyl-6-Phenyl-4(3H)-pteridinones and their 8-Oxides. Potential Substrates of Xanthine Oxidase
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Synthetic routes for the preparation of 3-alkyl-6-phenyl-4(3H)-pteridinines 6 and their corresponding 8-oxides 5 (R=CH3, C2H5, (CH2)2CH3, (CH2)3CH3, CH(CH3)C2H5, CH(CH3)2 and CH(C2H5)CH2OCH(OC2H5)2) are described and their reactivities towards xanthine oxidase from Arthrobacter M-4 are determined.Only the 3-methyl derivative of 6-phenyl-4(3H)-pteridinone and its 8-oxide i.e. 6a and 5a are found to be substrates although their reactivities are still very low.Oxidation takes place at C-2 of the pteridinone nucleus.All the 3-alkyl derivatives are less tightly bound to t he enzyme than 6-phenyl-4(3H)-pteridinone.Introduction of the N-oxide at N-8 considerably lowers the binding of the substrates.Inhibition studies have revealed that 3-methyl-6-phenyl-4(3H)-pteridinone (6a) is non-competitive inhibitor with a Ki-value of 47 μM and the 3-ethyl derivative (6b) an uncompetitive one with a Ki-value of 19.6 μM.
- Meester, J. W. G. De,Kraus, W.,Plas, H. C. van der,Brons, H. J.,Middelhoven, W. J.
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p. 1109 - 1116
(2007/10/02)
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