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V.M. Monnier; Ornithine is a Novel Amino Acid in Human Lens Crystallins and Collagen. V. M. Monnier, D.R. Sell. Departments of Pathology and Biochemistry, Case Western Reserve University, Cleveland, OH . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1688.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Long–lived proteins such as those of the lens and the extracellular matrix undergo age–related postsynthetic modifications which destabilizes them by altering their conformation, charge and helicity, thereby enhancing their resistance toward proteolysis and propensity to aggregate and form amyloid. In addition, they acquire UV photosensitizers which catalyze oxidative damage. The unexpected finding of ornithine (ORN), the decarbamidation product of arginine and reportedly a nonprotein amino acid, in acid hydrolyzates of lens crystallins and skin collagen led us to investigate its source and mechanism of formation. Methods: In order to investigate if ORN was an artifact of acid hydrolysis, the proteins were submitted to a variety of modifications, one of which was reductive alkylation with formaldehyde to dimethyl–ornithine (DM–ORN). Glycated ornithine (G–ORN) as "furosine–ornithine" (FUR–ORN) and carboxymethylated ornithine (CM–ORN) were assayed in acid hydrolysates using appropriate standards. Standards of advanced glycation products (AGEs) of arginine were acid hydrolyzed and assayed for ORN. All assays were performed by liquid chromatography coupled to electrospray ionization mass spectrometry (LC/ESI–MS) with or without derivatization. Results: ORN in acid hydrolysates of human lens and skin proteins increased from approximately 1 to 15 nmol/mg protein from ages 10 to 90 yrs. With modification of the protein by reductive alkylation, DM–ORN increased with age from 0.5 to 15 and 0 to 5 nmoles/mg protein in lens and skin, respectively. CM–ORN and FUR–ORN increased with age in human skin from 0 to 30 and 0 to 180 pmol/mg protein, respectively. Acid hydrolysis of a variety of arginine–based AGEs showed yields of ORN increased in the following order: methylglyoxal (MG) and glyoxal (G)–derived imidazoline crosslinks (MODIC < GODIC) < glucosepane < hydroimidazolones (MG–H1 < G–H1) < pentosidine < argpyrimidine. Conclusions:The age–related increase in DM–ORN, CM–ORN and FUR–ORN provides evidence for the in vivo formation of ORN in aging human tissue proteins. The exact mechanism of ORN formation is unclear, but the data suggest arginine–based AGE products might serve as precursors for the in vivo formation of ornithine from arginine. The appearance of ORN in aging human proteins might represent a compensatory mechanism aimed at regenerating the protein charge that is lost upon in vivo modification of ARG by carbonyl stressors of the Maillard reaction involving reducing sugars, oxoaldehydes or ascorbate.
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