Identification of isomeric 5-hydroxytryptophan- and oxindolylalanine-containing peptides by mass spectrometry

5-Hydroxytryptophan (5-HTP)-induced intracellular syndrome in mouse non-neural embryonic cells is associated with inhibited proliferation and cell death

Biogenic monoamines are involved in the regulation of various processes in both neural and non-neural cells during development. The present study aimed to identify the regulatory effects of serotonin (5-HT) and its precursors (l-tryptophan and 5-hydroxytryptophan, 5-HTP) on proliferation and cell death in mouse embryonic stem cells (ESCs) and embryonic fibroblasts (MEFs and 3T3 cells). The concentration-dependent cell growth and viability of the ESCs, MEFs and 3T3 cells were analyzed after treatment with l-tryptophan, 5-HTP and 5-HT in the concentration range 10^-6 - 10^-2 M. Treating the cells with 5-HTP, but not l-tryptophan and 5-HT, induced reversible toxic effects. 5-HTP treatment (10^-3 - 10^-2 M) significantly inhibited cell proliferation through blocking of the S-phase of the cell cycle and increasing apoptotic and necrotic cell death. Moreover, 5-HTP treatment stimulated a reorganization of the actin and tubulin networks and upregulated the gene expression of enzymes involved in 5-HT synthesis and metabolism: aromatic amino acid decarboxylase (Aadc/Ddc), monoamine oxidase A (Maoa), and transglutaminase 2 (Tgm2). HPLC analysis found no changes in the intracellular and extracellular levels of 5-HT after 5-HTP treatment, but a significant increase of intracellular 5-HTP levels. However, inhibition of AADC with NSD-1015 or transglutaminase with cystamine prevented 5-HTP-induced cell growth impairment and attenuated the toxic effects of 5-HTP treatment. Our results suggest that 5-HTP can induce toxic effects through cell cycle arrest and cell death in embryonic stem and somatic cells by enhancing the levels of 5-HT-mediated protein modifications. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Fingerprinting desmosine-containing elastin peptides

Elastin is a vital protein of the extracellular matrix of jawed vertebrates and provides elasticity to numerous tissues. It is secreted in the form of its soluble precursor tropoelastin, which is subsequently cross-linked in the course of the elastic fiber assembly. The process involves the formation of the two tetrafunctional amino acids desmosine (DES) and isodesmosine (IDES), which are unique to elastin. The resulting high degree of cross-linking confers remarkable properties, including mechanical integrity, insolubility, and long-term stability to the protein. These characteristics hinder the structural elucidation of mature elastin. However, MS(2) data of linear and cross-linked peptides released by proteolysis can provide indirect insights into the structure of elastin. In this study, we performed energy-resolved collision-induced dissociation experiments of DES, IDES, their derivatives, and DES-/IDES-containing peptides to determine characteristic product ions. It was found that all investigated compounds yielded the same product ion clusters at elevated collision energies. Elemental composition determination using the exact masses of these ions revealed molecular formulas of the type CxHyN, suggesting that the pyridinium core of DES/IDES remains intact even at relatively high collision energies. The finding of these specific product ions enabled the development of a similarity-based scoring algorithm that was successfully applied on LC-MS/MS data of bovine elastin digests for the identification of DES-/IDES-cross-linked peptides. This approach facilitates the straightforward investigation of native cross-links in elastin.