Guanine hypoxanthine phosphoribosyl transferase

The answer is c. (Katzung, p 933.) Resistance to thioguanine occurs because of an increase in alkaline phosphatase and a decrease in hypoxanthine-guanine phosphoribosyl transferase. These enzymes are responsible, respectively, for the increase in dephosphorylation of thiopurine nucleotide and the conversion of thioguanine to its active form, 6-thioinosinic acid. 

There was no increase in mutation frequency at the hypoxanthine-guanine phosphoribosyl transferase gene locus in the presence or absence of S9 (Bootman et al. 1988b), and results were negative in a DNA repair assay with E. coli (Hodson-Walker and May 1988). 

DNA = Deoxyribonucleic acid HGPRT = hypoxanthine-guanine phosphoribosyl transferase RNA = Ribonucleic acid -= negative result + = positive result (+) = weakly positive result... 

Brimer PA, Tan EL, Hsie AW. 1981. Effect of metabolic activation on the mutagenicity and cytotoxicity of ethylene dibromide in the Chinese hamster ovary hypoxanthine guanine phosphoribosyl transferase [Abstract]. Environ Mutagen 3 317-318. 

Fig. 13.1 Pathways of thiopurine metabolism. The positions of two polymorphically expressed enzymes, TPMT (thiopurine methyl transferase) and ITPA (inosine triphosphate pyrophosphatase), are shown. HGPRT, hypoxanthine guanine phosphoribosyl transferase 6-TIDP, 6-thioi-nosine diphosphate 6-TIMP, 6-thioinosine monophosphate 6-TITP, 6-thio inosine trinophosphate...

GMP NADH Disappearance GMP Kinase, Pyruvate Kinase, and Lactate Dehydrogenase Hypoxanthine-Guanine Phosphoribosyl-transferase ... 

Rare hereditary deficiency Avoid in patients with rare hereditary deficiency of hypoxanthine-guanine phosphoribosyl-transferase (HGPRT), such as Lesch-Nyhan and Kelley-Seegmiller syndromes. 

Amacher, D.E. Zelljadt, I. (1984) Mutagenic activity of some clastogenic chemicals at the hypoxanthine guanine phosphoribosyl transferase locus of Chinese hamster ovary cells. Mutat. Res., 136, 137-145... 

Inherited deficiency in hypoxanthine-guanine phosphoribosyl-transferase... 

Cultivate a suitable malignant myeloma cells deficient in HPGRT (hypoxanthine guanine phosphoribosyl transferase), which is a genetic marker for the selection of the hybrid cells after fusion. 

Several bacterial and mammalian short-term tests for genetic toxicity as well as their biochemical and genetic rationale are described in Chapter 21 on toxicity testing. They include the salmonella assay, the Chinese hamster ovary cell/hypoxanthine-guanine phosphoribosyl transferase assay, the mouse lymphoma assay, the mammalian transformation assay, sister chromatid exchange, and the chromosome aberration assay. 

Fig. 3.2 Hybridoma technique for the production of monoclonal antibodies. Spleen (milt) cells, which have been taken from mice (being immunized with an antigen X) contain anti-X-antibody-producing B cells. These cells are fused with myeloma cells in the presence of polyethylene glycol (PEG) and then taken to the HAT (hypoxanthine-aminopterin-thymi-dine) medium. HAT will induce death to myeloma cells because of the absence of the enzyme hypoxanthine-guanine-phosphoribosyl-transferase (HGPRT). Hybridoma cells, how-...

G Vasanthakumar, RL Davis Jr, MA Sullivan, JP Donahue. Cloning and expression in Escherichia coli of a hypoxanthine-guanine phosphoribosyl transferase-encoding cDNA from Plasmodium falciparum. Gene 91 63-69, 1990. 

One system uses mouse lymphoma cells and detects mutations that cause deficiency of thymidine kinase (TK). Another uses Chinese hamster cells and detects mutations in the gene that produces hypoxanthine-guanine phosphoribosyl transferase (HGPRT). Both tests cure efficient, are widely applied, and can be completed in a few weeks. Although not as simple, rapid, and efficient as the Salmonella tests, they have the advantage of being done in a eukaryote. Mammalian-cell cultures cure also used to test for chromosomal mutation. 

HGPRT Hypoxanthine-guanine phosphoribosyl transferase (also called HPRT) an enzyme involved in the utiliza-... 

Single dose cynomolgus monkeys Repeat dose mice, rats, cynomolgus monkeys Fertility and early embryonic development rats Developmental rats, mice, rabbits Prenatal and postnatal development rats Genetic toxicology Ames reverse mutation, Chinese hamster ovary hypoxanthine guanine phosphoribosyl-transferase forward point mutation assay, cytogenetics in human lymphocytes... 

The assay was described by Clive and co-workers (Clive et al. 1972) as a mutational assay system using the TK locus in mouse lymphoma cells. In the following years, he and his collaborators undertook a large-scale of investigation of the potential and optimal conduct of the assay. This included the use of the above mentioned TFT to select tk mutants, a comparison of the hypoxanthine guanine phosphoribosyl transferase (hprt) and tk loci, an analysis of the best expression time for tk mutant selection and a description of distinct large and small colony tk mutants. 

The loci that can typically be used for mutation assessment are ones that can be selected for, although PCR methods currently available and under development will allow for analysis of any gene for which the DNA sequence is known. The selection procedure requires that a locus be heterozygous or hemizygous such that only a single mutation is required for the selectable phenotype to be assessed. Examples of such loci are the hprt (hypoxanthine-guanine phosphoribosyl transferase) gene... 

CD-I mice were exposed to purified air or benzene by inhalation at 0.04, 0.1, or 1.0 ppm for 22 hours per day, 7 days per week for 6 weeks (Ward et al. 1992). The effects of in vivo exposure to benzene were evaluated by using an autoradiographic assay to determine the frequency of mutants which represent mutations at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus in spleen lymphocytes. At the end of the six weeks exposure period, lymphocytes were recovered from the spleens of the mice and cryopreserved prior to assay. Mutant cells were selected on the basis of their ability to incorporate tritiated thymidine in the presence of 6-thioguanine. The increased frequencies of mutant spleen lymphocytes were significant at the low and mid, but not the high dose, and the method does not take into account possible clonal expansion. Further evaluation of the induction of gene mutation at these dose levels seems warranted. 

Gene mutation tests in a eukaryotic system in vitro, e.g. the Hypoxanthine - guanine - phosphoribosyl - transferase (HGPRT) gene mutation tests in hamster cells. 

Potentially serious side-effects of azathioprine that are dose- and duration-dependent are haematological (leukopenia and thrombocytopenia) and gastrointestinal. Azathioprine is metabolized to 6-mercaptopurine, and both compounds are rapidly eliminated from blood and oxidized in red cells and liver. Activation of 6-mercaptopurine occurs via hypoxanthine-guanine phosphoribosyl-transferase. 6-Mercaptopurine is inactivated by thiopurine S-methyltransferase and by xanthine oxidase. 

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