Xanthine oxidase mammalian cells

One may mention the relative lack of information on the possible toxicity mechanisms of other groups of explosives. The administration of hexahydro-l,3,5-trinitro-l,3,5-triazine (RDX 30-300 mg kg-1 daily for 13 weeks) to rats caused hypotriglycidire-mia, convulsions, and death [4], In contrast, pentaerythritol tetranitrate (PETN 0.5%-1.0% in standard diet for 13 weeks) was nontoxic to rats [80], RDX was much less cytotoxic to V79 and TK-6 mammalian cell cultures than TNT [9], There also are very few data on their reactions with mammalian enzymes. Rabbit liver cytochrome P-450 2B4 (EC 1.14.14.1) converted RDX into 4-nitro-2,4-diazabutanal, two nitrite ions, ammonium, and formaldehyde, consuming one equivalent NADPH [81]. However, it is unclear whether this slow reaction (kcat < 0.01 s-1) may contribute to the toxicity of RDX. Xanthine oxidase transformed octohydro-l,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) at a lower rate, 10.5 nmol h 1 mg 1 protein under anaerobic conditions, into nitrite, formaldehyde, nitrous oxide, formic acid, and ammonium [82], Our preliminary observations show that RDX was much less reactive substrate for P-450R and E. cloacae NR than NTO or ANTA [53], Thus, the mechanisms underlying toxicity of RDX remain undisclosed. 

In Chapter 1, the structure, synthesis and toxicity of ricin, a protein present in the endosperm cells of the seeds of the castor oil plant, are described and possible uses of this toxic agent in immunology as conjugates with antibodies are discussed. The biochemical and pharmacological evidence for the presence of functional histamine receptors in the mammalian central nervous system is set out in Chapter 2. Chapter 3 covers the chemical properties of molybdenum-containing enzymes such as aldehyde oxidase and xanthine oxidase, although their physiological roles are by no means clear. 

Pyrazolopyrimidines are purine analogues in which the nitrogen and carbon of the imidazole ring are inverted. The best known member of this group, allopurinol (4-hydroxypyrazolo(3,4-d)pyrimidine HPP), is a structural analogue of hypoxanthine. Its major metabolic conversions in mammalian cells are to oxipurinol, via xanthine oxidase, and to allopurinol ribonucleoside (HPPR)l. 

Dideoxynebularine (2 ,3 -dideoxypurine nucleoside, ddPN) 1 was one of a number of C-6 modified purine dideoxynucleosides synthesized in our laboratory (Scheme 2). It was designed to be a prodrug of ddl and was synthesized from 5 -protected ddA by reductive deamination using a procedure previously developed by us, followed by deprotection. Compound 1 was found to be inactive against the cytopathic effect of HIV-l and HIV-2 in MT-4 cells. It was not toxic to MT-4 cells at 200 jiM. It was not a substrate for xanthine oxidase. In contrast, C-6 substituted purine dideoxynucleosides that are substrates for mammalian ADA, show anti-HIV activity [e.g., 6-iodo ddPN, 2, synthesized from 5 -protected ddA by radical deamination/halogenation (n-pentyl nitrite, CH,L, CHjCN, A), followed by deprotection]. 

In other experiments, lines of mammalian cells from several species, including man and rodents, were grown in the presence of compounds known to induce xanthine oxidase in a eucaryotic fungus (Aspergilus nidulans), Uric acid and hypoxanthine were the main inducers tested, and both were shown to enter the cultured cells. However, neither of these compounds led to an increase in the xanthine oxidase activity of the mammalian cells. 

These results are reported more extensively in the following paper Brunschede, H. and Krooth, R. S. (1973). "Studies on the xanthine oxidase activity of mammalian cells." Biochem. Genetics (in press). 

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