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 provi des a signal for the degradation of proteins. Despite this, the AGE wh ich act a recognition factor for receptor-mediated endocytosis and deg radation of glycated proteins by monocytes and macrophages has not bee n identified. Methylglyoxal, a reactive alpha-oxoaldehyde and physiolo gical metabolite, reacted irreversibly with arginine residues in prote ins to form elta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine and N-d elta-(5-methyl-4-imidazolon-2-yl)ornithine residues. Human serum album in minimally-modified with methylglyoxal (MG(min)-HSA) was bound by ce ll surface receptors of human monocytic THP-1 cells in vitro at 4 degr ees 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). elta-(5-hydro -5-methyl-4-imidazolon-2-yl)ornithine displaced MG(min)-HSA from THP-1 cells, suggesting that the elta-(5-hydro-5-methyl-4-imidazolon-2-yl)o rnithine residue was the receptor recognition factor. At 37 degrees C, MG(min)-HSA was internalised by THP-1 cells and degraded. Similar bin ding and degradation of human serum albumin modified by glucose-derive d AGE was found but only when highly modified. MG,,,-HSA, therefore, i s the first example of a protein minimally-modified by AGE-like compou nds that binds specifically to monocyte receptors. The irreversible mo dification of proteins by methylglyoxal is a potent signal for the deg radation of proteins by monocytic cells in which the arginine derivati ve, elta-(5-hydro-5-methyl-4-imidazolon-2-yl)ornithine is the receptor recognition factor. This factor is not present in glucose-modified pr oteins.