Puriiniset

Puriiniset are a family of heterocyclic aromatic compounds with a fused imidazole-pyrimidine ring system. In biology the most important purines are adenine and guanine, which serve as the two bases in DNA and RNA. When linked to a sugar, they form nucleosides (adenosine, guanosine); when those are phosphorylated they give nucleotides (AMP, ADP, ATP, GMP, GDP, GTP). Purine derivatives include widely used alkaloids and drugs such as caffeine, theobromine, and many purine analogs used in medicine.

Purine metabolism comprises two major pathways: de novo biosynthesis and salvage. De novo synthesis builds the purine ring from small precursors on ribose-5-phosphate, with PRPP as a key activated sugar donor. Salvage pathways recycle free bases back into nucleotides, mainly via hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and adenine phosphoribosyltransferase (APRT). In humans, the degradation of purines ends with uric acid, which is excreted; many other organisms further oxidize uric acid to allantoin.

Purines are essential for genetic information storage and transmission, as well as cellular energy and signaling. ATP and GTP power many reactions; cyclic AMP and cyclic GMP act as second messengers; purine-containing cofactors such as NAD+ and FAD participate in redox chemistry and metabolism.

Clinical relevance includes disorders of purine metabolism and their treatment. Gout results from excess uric acid; Lesch-Nyhan syndrome arises from HGPRT deficiency; ADA deficiency causes severe combined immunodeficiency. Pharmacologically, purine analogs such as 6-mercaptopurine and 6-thioguanine are used as chemotherapy agents; xanthine oxidase inhibitors like allopurinol lower uric acid levels.