[Adenine phosphoribosyltransferase (APRT)].

Adenine phosphoribosyltransferase (APRT) is a purine metabolic enzyme that salvages adenine moiety generated via the polyamine synthetic pathway. Adenine is produced by the cleavage of methylthiodenosine (MTA), a by-product of the polyamine synthesis. None of the normal somatic cells but about 23% of human malignant cells lack the enzyme MTA phosphorylase because of the homozygous deletion of 9p21 and are deficient in the salvage route of the adenine molecule. APRT gene is located at 16q24, is about 2200 bp long and is composed of 5 exons. The cDNA codes for 180 amino acids which include a PRPP binding domain. The Met at the N-terminal is removed and the Ala is next acetylated. The protein is cleaved into two peptides at amino acid 136 (Met) by BrCN treatment and the N-terminal peptide includes the PRPP binding domain. Although most of the somatic cells express APRT gene, some cells do not, even in normal subjects. This is because of somatic mutations. APRT locus is one of a few genetic loci where human somatic mutations can be observed. We can observe loss of heterozygosity (LOH) and point mutational events at the somatic cell level at this locus. The mode of the somatic mutations is quite similar to that observed in neoplastic cells at the tumor-suppressor loci.