Quantitative Serum Proteomics of Tryptophan Nutrition Using Bi-directional Heavy Oxygen Labeling with a RuBisCO Internal Standard

Tryptophan plays an important role in vertebrate metabolism as not only a building block of proteins, but also as a precursor of serotonin, melatonin, niacin and kynurenines, which influence immune tolerance. Here we use an animal paradigm and quantitative serum proteomics to model tryptophan deficiency. We applied bidirectional H2 16/18O labeling to serum proteins from chickens fed either a tryptophan-deficient or- adequate diet and used the plant protein RuBisCO as an internal standard. The proteins were trypsin digested and processed by 2-dimensional liquid chromatography electrospray ionization tandem mass spectrometry (2D LC ESI MS2). The resulting mass spectra were analyzed using the SEQUEST algorithm and the ProteinMapper program to identify proteins that had increased or decreased expression. We identified 4161 proteins labeled bidirectionally, of which 46 were increased and 90 decreased (~3%). Using Ingenuity Pathways Analysis (IPA) software, we found that a tryptophan nutritional deficiency may affect not only the immune and neurological systems, but our modeling also suggests that it may be important in cancer, optic atrophy and cardiomyopathy.