Methylglyoxal-modified arginine residues - A signal for receptor-mediated endocytosis and degradation of proteins by monocytic THP-1 cells

Non-enzymatic glycosylation or glycation of proteins to form advanced glycation endproducts (AGE) has been proposed as a process which provides a signal for the degradation of proteins. Despite this, the AGE which act a recognition factor for receptor-mediated endocytosis and degradation of glycated proteins by monocytes and macrophages has not been identified. Methylglyoxal, a reactive alpha-oxoaldehyde and physiological metabolite, reacted irreversibly with arginine residues in proteins to form N-delta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine and N-delta-(5-methyl-4-imidazolon-2-yl)ornithine residues. Human serum albumin minimally-modified with methylglyoxal (MG(min)-HSA) was bound by cell surface receptors of human monocytic THP-1 cells in vitro at 4 degrees C: the binding constant K-d value was 377 +/- 35 nM and the number of receptors per cell was 5.9 +/- 0.2 x 10(5) (n = 12). N-alpha-Acetyl-N-delta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine displaced MG(min)-HSA from THP-1 cells, suggesting that the N-delta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine residue was the receptor recognition factor. At 37 degrees C, MG(min)-HSA was internalised by THP-1 cells and degraded. Similar binding and degradation of human serum albumin modified by glucose-derived AGE was found but only when highly modified. MG,,,-HSA, therefore, is the first example of a protein minimally-modified by AGE-like compounds that binds specifically to monocyte receptors. The irreversible modification of proteins by methylglyoxal is a potent signal for the degradation of proteins by monocytic cells in which the arginine derivative, N-delta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine is the receptor recognition factor. This factor is not present in glucose-modified proteins.