The amino acids involved in purine synthesis are glycine, aspartic acid, and glutamine. There are two pathways of purine synthesis, one is ab initio synthesis and the other is remedial synthesis. 1. Ab initio synthesis: Purine nucleotides are synthesized using amino acids, one-carbon units, CO₂, amino acids, and ribose phosphate as raw materials. The main sites of synthesis are in: liver, small intestine, thymus. There are four nitrogen atoms on the purine ring, two of which are provided by glutamine and the other two by aspartic acid and glycine. 2. Remedial synthesis: synthesized using free purines or purine nucleosides in the body, the brain and bone marrow can only synthesize purines by the remedial synthesis pathway. This process saves the consumption of amino acids and energy during scratch synthesis.