Histamine, a component of the immune system whose overproduction results in an allergic response, is derived from histidine by decarboxylation. Tyrosine is a precursor of the hormones thyroxine (tetraiodothyronine), which controls the rate of metabolic processes throughout the body, and epinephrine (adrenaline), the “fight or flight” hormone. Epinephrine is metabolized to 5,6-dihydroxyindole, which is a precursor of melanin, a complex polymeric pigment that is formed on exposure to sunlight and is the chemical basis of tanning. The neurotransmitter serotonin (5-hydroxytryptamine) and the nicotinamide ring of NAD⁺ are synthesized from tryptophan. The reactive terminus of sphingosine, an intermediate in the synthesis of sphingolipids, comes from serine. We now consider in more detail especially prominent biochemicals derived from amino acids—the nucleotide bases. Purines and pyrimidines are derived largely from amino acids and are the precursors of DNA, RNA, and various coenzymes.

We begin by examining nucleotide synthesis. The initial products of nucleotide synthesis are ribonucleotides, and we will see how ribonucleotides are converted into deoxyribonucleotides, the substrates for DNA synthesis. We then turn to the regulation of nucleotide synthesis. Finally, we consider pathological conditions that result from perturbations in nucleotide metabolism.