Methylglyoxal

METHYLGLYOXAL, propanonal, pyruvic aldehyde, C₃H₄O₂=CH₃CO-CHO, the simplest representative of ketone-aldehydes. It can be considered a derivative of the simplest dialdehyde, glyoxal, CHO-CHO. It is known in a monomolecular form (an intensely yellow, mobile liquid with a pungent odor) and in the form of polymers consisting of three or four molecules. The trimolecular form (C₃H₄O₂)₃ is a thick, glycerin-like yellowish liquid; upon dissolution in water, it gradually converts into the monomeric form. It is obtained by distilling a solution of dioxyacetone acidified with sulfuric acid or, better, by oxidizing acetone with selenium anhydride and distilling the resulting product at 13 mm and 52°. It yields a characteristic bis-hydrazone with 2,4-dinitrophenylhydrazine, with a melting point of 298°, which is insoluble in a soda solution and produces an intense blue-violet coloration with alcoholic alkali. The latter reaction is used for the colorimetric quantitative determination of methylglyoxal; it can also be determined iodometrically. For a long time, methylglyoxal was considered the central link in the transformations of carbohydrates in living cells and tissues during fermentation and glycolysis. This was supported, on the one hand, by the wide distribution of the enzyme glyoxalase (methylglyoxalase, ketone-aldehyde-mutase) in almost all examined cells and, on the other hand, by its actual detection in amounts approaching theoretical levels (up to 85%) during the breakdown of carbohydrates under certain conditions, under the action of enzymes of both plant and animal origin. It was assumed that the formation of lactic acid occurs via the "internal dismutation" of the hydrate form of methylglyoxal, and during alcoholic fermentation, this hydrate undergoes dehydrogenation, yielding pyruvic acid as a further intermediate product. The conversion of methylglyoxal into lactic acid proceeds only in the presence of glutathione in its reduced form as a coenzyme. Recent studies (Embden, Meyerhof) suggest that the desmolytic breakdown of carbohydrates proceeds mainly not through methylglyoxal, but through the formation of phosphorylated trioses, phosphoglyceric aldehyde, and phosphodioxyacetone. The detection of methylglyoxal in previous experiments is explained by the fact that it can easily form as a result of the secondary transformation of these products; in an acidic medium, this transformation is completed at ordinary temperatures in 10 minutes. Even if one does not attribute the role of the main intermediate product of carbohydrate breakdown to methylglyoxal, it is still premature to completely deny its biological significance, especially if one takes into account the already noted fact of the wide distribution of an enzyme acting specifically on methylglyoxal. However, we do not yet possess information on the extent to which, and in what cases, the pathway of sugar breakdown via methylglyoxal is utilized.