Guanine-phosphoribosyl

Guanine Phosphoribosyl Transferase. Guanine phosphoribosyl transferase (GPRT) is one of the enzymes of the purine salvage pathway, which is needed by protozoa because they lack the ability to synthesize purine nucleotides. 

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. 

In many cells, the capacity for de novo synthesis to supply purines and pyrimidines is insufficient, and the salvage pathway is essential for adequate nucleotide synthesis. In patients with Lesch-Nyhan disease, an enzyme for purine salvage (hypoxanthine guanine phosphoribosyl pyrophosphate transferase, HPRT) is absent. People with this genetic deficiency have CNS deterioration, mental retardation, and spastic cerebral palsy associated with compulsive self-mutilation, Cells in the basal ganglia of the brain (fine motor control) normally have very high HPRT activity. These patients also all have hyperuricemia because purines cannot be salvaged. 

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. 

The mouse lymphoma assay is in fact just one of several mammalian cell assays designed to determine increases in mutation rate. It focuses on the thymidine kinase (tk) assay in murine lymphoma cells (L5178Y), though tk data have also been produced from the human lymphoblastoid cell line TK6, and at the hgprt (hypox-anthine-guanine phosphoribosyl-transferase) locus in Chinese Hamster ovary or lung (V79) cells and mouse lymphoma cells. Since Ames mutation data and often... 

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... 

Mulligan, R. C and Berg, P. (1981) Selection for animal cells that express the Escherichia coli gene coding for xanthine-guanine phosphoribosyl transferase Proc Natl. Acad. Sci USA 78, 2072—2076. 

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. 

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