Chemical and biological determinants of the intramolecular carbon isotopic composition of amino acids in some organisms

Most biochemical reactions have isotopic consequences; however unless a reaction influences the primary formation of organic matter it rarely has a measurable effect on the isotopic composition of bulk carbon. The isotopic influence of most reactions in metabolism extends only to the carbon atoms participating in the reaction in question. Only by examining the isotopic composition of the carbon atoms participating in the reaction can its effect be measured in vivo.; In this research, an isotopic probe of TCA cycle activity and amino acid synthesis was developed, and some applications to understanding of the biochemical, physiological, and environmental milieu of amino acid synthesis within a variety of organisms were explored. The isotopic composition of the α-carboxyl carbons of amino acids, which are derived in part from carbon fixed by the anapleurotic pathway, was employed as an indicator of amino acid synthesis within organisms. Conceptual models of carbon fluxes during primary metabolism led to contrasting predictions for the relative 13C enrichment of carboxyl carbons fixed during autotrophic and heterotrophic metabolism.; The 13C enrichment of the amino acid carboxyl carbon fraction of organic matter was used to define the locations and pathways of amino acid synthesis within organisms in a variety of settings. Among autotrophs, variations in the relative 13C enrichment of carboxyl carbons were used to define the operation of different anapleurotic carboxylating enzymes among plant taxa. Within-plant amino acid source/sink relationships in Spartina alterniflora were inferred from isotopic contrasts between autotrophic (leaf) and heterotrophic (root and rhizome) plant tissues. In heterotrophic organisms, the relative importance of dietary assimilation vs. de novo amino acid synthesis was estimated from the deviation of isotopic values from model predictions. Industrial sources of monosodium glutamate produced by glutamate-fermenting microorganisms could be defined by the relative 13C depletion of the carboxyl carbon fraction.; The results demonstrate the utility of intramolecular isotopic measurements as a probe into the metabolic properties of organisms. By focusing directly on the carbon involved in particular reactions, some of the details of carbon allocation and flux within organisms may be revealed.