Isotopically labeled branched-chain amino acid

Additional or different labeled and unlabeled standards can be used. However, note that the incubation as described here includes isotopically labeled L-valine and L-isoleucine and that the metabolites of these branched chain amino acids can interfere with isotopically labeled internal standards for butyrylcarnitine or propi-onylcarnitine. 

Pioneering tracer studies carried out in the laboratories of D. J. Robins and I. Spenser had demonstrated that the carbon skeleton of the necine base originates from ornithine or arginine via putrescine. Extension of these studies in the 1980s using precursors labeled with stable isotopes confirmed that retronecine is derived from putrescine via a symmetrical intermediate such as homospermidine [17-19]. The formation of the necic acids is less well understood. Early tracer-studies primarily carried out by D. H. G. Grout and collaborators revealed that the carbon skeletons of all aliphatic necic acids so far studied are derived from branched-chain amino acids, i. e., isoleucine, and less frequently leucine and valine [see 17]. 

In vitro testing using cultured skin fibroblasts with stable isotope-labeled substrates can be helpful in elucidating and confirming some defects of fatty acid [12] and branched-chain amino acid [13] catabolism. 

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