Methylglyoxal formation in rat liver cells.

The formation of methylglyoxal from triose phosphates was studied in dialyzed whole cell homogenates of rat liver. In the cell-free system, methylglyoxal is largely derived from dihydroxyacetone phosphate (DHAP). The addition of the cofactor of glyoxalase activity, reduced glutathione (GSH), to the system inhibits the accumulation of methylglyoxal from DHAP, whereas the absence of GSH promotes the accumulation of methylglyoxal regardless of the presence or absence of NAD. To determine whether methylglyoxal synthase activity (EC 4.2.99.11) is present in the intact cell system, methylglyoxal formation from xylitol and glucose was also studied in a single cell suspension of liver of rats fasted for 48 h. Rat liver cells utilize glyceraldehyde 3phosphate as a precursor of methylglyoxal, whereas DHAP is converted to a lesser degree, probably as a result of lack of permeability through the cell membrane. Methylglyoxal is converted to pyruvate and Llactic acid by a-ketoaldehyde dehydrogenase activity, and to D-lactic acid formation through the glyoxalase system. The conversion and dilution of w-’4C]xylitol and [U-’4C]glucose to [‘4C]methylglyoxal and [14C]pyruvate was quantitated in the presence and absence of NaF and diamide through the formation of the 2,4dinitrophenylhydrazones of methylglyoxal and pyruvate. In the presence of inhibitors, the conversion of the substrates into [’4C]methylglyoxal is less than 0.4% from glucose and as much as 4% from xylitol without dilution of the specific activity in either. It is concluded that methylglyoxal formation in rat liver cells is a metabolic process.