Hypoxanthine-guanine phosphoribosyltransferase
- OMIM: 308000
- MGI: 96217
The hypoxanthine phosphoribosyl transferase 1 ( HPRT1 ) is an enzyme in the purine metabolism of eukaryotes. In humans, changes to HPRT1 can lead to metabolic disorders such as Lesch -Nyhan syndrome and Kelley - Seegmiller syndrome due to mutations. The function of the enzyme is very sensitive to changes in the gene, so the gene is used in a cell line as a target for mutation testing.
Synthesis
HPRT1 the gene consists of 10 exons which are distributed over a 40- kilobase portion of the X chromosome. After translation and removal of the first methionine is a 217 amino acids and 24 kDa protein. Four of these molecules bind with two magnesium ions (one only loosely ) as cofactors for operatively HPRT1 enzyme which is localized in the cytoplasm.
Catalyzed reaction
The purine bases adenine, hypoxanthine, xanthine and guanine can be recycled by being constructed with a Phosphoribosylrest back to the nucleotide using two enzymes (HPRT and APRT ). This is very useful, since it first of ATP can be saved ( cost the formation of a purine at least 7 (AMP ) and 9 (GMP ) -energy phosphate bonds) and secondly the formation of uric acid is drastically reduced (so-called salvage pathway )
Pathology
There are a large number of changes in the HPRT gene is known, wherein so-called repeats ( retries of a base ), are rare. Mainly there are insertions, deletions and point mutations on the gene. In half of the altered gene products, the size is changed, the rest are changes to an amino acid.
The function of HPRT sensitive to changes in the amino acid sequence. Minor restrictions of the function cause similar symptoms such as gout and are referred to as Kelley - Seegmiller syndrome. All severe metabolic diseases are grouped under the Lesch -Nyhan syndrome.
Mutation test
When HPRT gene mutation test any mutants are detected by the loss of activity of the enzyme. Usually the hamster lung cell line ( V79 cells, ATCC no. CCL -93 ) is used for the test.
The selection of the mutants due to the different degrees of toxicity of the synthetic purine 6-thioguanine (TG ) for the mutant and the unchanged cells. TG is converted into cells with a functional HPRT in such toxic nucleotides that this die. The loss of HPRT activity by mutagenic substances, however, leads to resistance to TG and thus the survival and enrichment of cells with mutated HPRT. This can be quantified microscopically.