Methylglyoxal-induced glycation affects protein topography

Methylglyoxal is a metabolic byproduct that is elevated in diabetic tissue. We examined the effects of methylglyoxal on cytosolic aspartate aminotransferase (cAAT), which is an enzyme previously shown to be modified by glyceraldehyde, acrolein, and ribose 5-phosphate. In the present study we observed that methylglyoxal caused real-time changes in tryptophan (intrinsic) fluorescence. Millimolar concentrations of methylglyoxal predominately decreased the fluorescence emission at 388 nm. While micromolar concentrations also decreased emission at 388 nm, low levels of methylglyoxal caused a prominent redshift in the wavelength of maximal emission. The changes in intrinsic fluorescence reflect definable changes in protein topography. These observations are consistent with a change in conformation that is more compact than that of native cAAT, suggesting that intramolecular cross-links (i.e., lysine-lysine) or hydrophobic pockets (i.e., carboxyethyl-lysines) were formed. Methylglyoxal also inhibited activity, and the inhibition correlated with the methylglyoxal-induced change in protein conformation.