The stable isotope composition of amino acids refers to the abundance of heavy and light non-radioactive isotopes of carbon (C and C), nitrogen (N and N), and other elements within these molecules. Amino acids are the building blocks of proteins. They are synthesized from alpha-keto acid precursors that are in turn intermediates of several different pathways in central metabolism. Carbon skeletons from these diverse sources are further modified before transamination, the addition of an amino group that completes amino acid biosynthesis. Bonds to heavy isotopes are stronger than bonds to light isotopes, making reactions involving heavier isotopes proceed slightly slower in most cases. This phenomenon, known as a kinetic isotope effect, gives rise to isotopic differences between reactants and products that can be detected using isotope ratio mass spectrometry. Amino acids are synthesized via a variety of pathways with reactions containing different, unknown isotope effects. Because of this, the C content of amino acid carbon skeletons varies considerably between the amino acids. There is also an isotope effect associated with transamination, which is apparent from the abundance of N in some amino acids.
Because of these properties, amino acid isotopes record useful information about the organisms that produce them. Variations in metabolism between different taxonomical groups give rise to characteristic patterns of C enrichment in their amino acids. This allows the sources of carbon in food webs to be identified. The isotope effect associated with transamination also makes amino acid nitrogen isotopes a useful tool to study the structure of food webs. Repeated transamination by consumers results in a predictable increase in the abundance of N as amino acids are transferred up food chains. Together, these application, among others in ecology, demonstrate the utility of stable isotopes as tracers of environmental processes that are difficult to measure directly.