Interaction of Methylglyoxal with alpha-lactalbumin, beta-lactoglobulin and alpha S casein – A Comparative Study.

Methylglyoxal (MG) is a highly reactive alpha-oxoaldehyde, which increases under diabetic condition. It is known to form Advanced Glycation Endproducts (AGEs) by non-enzymatic glycation following the Maillard reaction. The nature of interaction of methylglyoxal, with milk proteins – alpha-lactalbumin (with substantial amounts of alpha helix, beta sheet and random coil), beta lactoglobulin (a major beta sheeted protein) and alpha S casein (a random coiled protein) have been studied using different spectroscopic techniques at low concentrations ranging from 0-20 micromolar. All the three milk proteins (10 micromolar) in absence and presence of methylglyoxal are incubated at 25oC for 3 and 6 days. UV absorption study indicates that methylglyoxal lowers the scattering above 300 nm observed for all the milk proteins compared to control. Fluorescence studies show that the intrinsic tryptophan fluorescence peak intensity increases upon incubation with methylglyoxal with all the three proteins but the tryptophan peak shifts to lower wavelengths only in case of beta lactoglobulin and alpha S casein. Slight decrease in the ANS fluorescence intensity is observed upon methylglyoxal binding to alpha lactalbumin and beta lactoglobulin whereas we see a slight increase in the intensity with alpha S casein. Far-UV CD spectra reveal that upon incubation of the milk proteins with methylglyoxal, the proteins tend to regain its secondary structure than controls (which were incubated under similar conditions in absence of methylglyoxal) and it is more pronounced in case of beta lactoglobulin and alpha S casein than alpha lactalbumin. Methylglyoxal reduces the hydrodynamic diameter of the proteins thereby increasing the compactness of the proteins. All the above studies indicate that methylglyoxal improves the stability of the proteins.