title: Nature and position of functional group on thiopurine substrates influence activity of xanthine oxidase--enzymatic reaction pathways of 6-mercaptopurine and 2-mercaptopurine are different.
abstract: Xanthine oxidase-catalyzed hydroxylation reactions of the anticancer drug 6-mercaptopurine (6-MP) and its analog 2-mercaptopurine (2-MP) as well as 6-thioxanthine (6-TX) and 2-thioxanthine (2-TX) have been studied using UV-spectroscopy, high pressure liquid chromatography, photodiode array, and liquid chromatography-based mass spectral analysis. It is shown that 6-MP and 2-MP are oxidatively hydroxylated through different pathways. Enzymatic hydroxylation of 6-MP forms 6-thiouric acid in two steps involving 6-TX as the intermediate, whereas 2-MP is converted to 8-hydroxy-2-mercaptopurine as the expected end product in one step. Surprisingly, in contrast to the other thiopurines, enzymatic hydroxylation of 2-MP showed a unique hyperchromic effect at 264 nm as the reaction proceeded. However, when 2-TX is used as the substrate, it is hydroxylated to 2-thiouric acid. The enzymatic hydroxylation of 2-MP is considerably faster than that of 6-MP, while 6-TX and 2-TX show similar rates under identical reaction conditions. The reason why 2-MP is a better substrate than 6-MP and how the chemical nature and position of the functional groups present on the thiopurine substrates influence xanthine oxidase activity are discussed.
authors: Department of Biotechnology, National Institute of Pharmaceutical Education and Research, Sector 67, SAS Nagar, Punjab, India. hemu_28@yahoo.com
authors: Hemlata
authors: H
authors: Tamta
authors: Sukirti
authors: S
authors: Kalra
authors: Ramasamy
authors: R
authors: Thilagavathi
authors: Asit K
authors: AK
authors: Chakraborti
authors: Anup K
authors: AK
authors: Mukhopadhyay
mesh_terms: D000818:Animals; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D013058:Mass Spectrometry / Q000379:methods; D015122:Mercaptopurine / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D015394:Molecular Structure; D011684:Purine Nucleosides / Q000737:chemistry*; D011687:Purines / Q000737:chemistry / Q000378:metabolism*; D013438:Sulfhydryl Compounds / Q000737:chemistry / Q000378:metabolism*; D013892:Thioxanthenes / Q000737:chemistry / Q000378:metabolism; D013997:Time Factors; D014969:Xanthine Oxidase / Q000378:metabolism*
chemical_list: D011684:Purine Nucleosides; D011687:Purines; D013438:Sulfhydryl Compounds; D013892:Thioxanthenes; C060355:2-thioxanthene; C520399:2-mercaptopurine; D015122:Mercaptopurine; D014969:Xanthine Oxidase