Nucleotides contain either a purine or a pyrimidine nitrogenous base. Quinine may target the malaria purine nucleoside phosphorylase enzyme. Adenosine is an endogenous purine nucleoside that modulates many physiological processes. The interchangeable nitrogenous nucleobases are derived from two parent compounds, purine and pyrimidine. Both purine and pyrimidine synthesis use phosphoribosyl pyrophosphate (PRPP) as a starting molecule. When purines are formed, they inhibit the enzymes required for more purine formation. The nitrogenous bases are classified into two parent compounds, purine and pyrimidine. Also, folate is essential for purine and pyrimidine base biosynthesis, so synthesis will be inhibited. Changing one purine or pyrimidine may change the amino acid that the nucleotides code for. Measured purine and pyrimidine compounds were found in the Murchison meteorite. The other purine nucleoside, guanosine, is cleaved to form guanine. Both purine and pyrimidine are self-inhibiting and activating. Transversions are replacement of a purine with a pyrimidine or vice versa. Ribavirin is a prodrug, which when metabolized resembles purine RNA nucleotides. Xanthosine monophosphate is an intermediate in purine metabolism.