Purine
A purine is a bicyclic organic compound. Two of the bases in nucleic acids, adenine and guanine, are purines. In DNA, these bases form hydrogen bonds with their complementary pyrimidines thymine and cytosine.
purine pyrimidine
A T
G C
In RNA, the complement of A is uracil instead of thymine:
purine pyrimidine
A U
G C
These hydrogen bonding modes are for classical Watson-Crick base pairing. Other hydrogen bonding modes are seen in both DNA and RNA, although the additional 2'-hydroxyl group of RNA expands the configurations through which RNA can form hydrogen bonds.
Some other purines are xanthine, hypoxanthine, and caffeine.
Purines from food (or from tissue turnover) are metabolized by several enzymes, including xanthine oxidase, into uric acid. High levels of uric acid can predispose to gout when the acid crystalizes in joints; this phenomenon only happens in humans and some animal species (e.g. dogs) that lack an intrinsic urease enzyme that can further degrade uric acid.
