Figure 4: Physiological defences against α,β-dicarbonyl precursors of glycation. α,β-dicarbonyls like methylglyoxal (R:CH3) and glyoxal (R:H) can be removed by aldose reductase-catalysed reduction to the corresponding alcohol or by oxidation to pyruvate and glycolate. respectively, catalysed by aldehyde dehydrogenase. Reactive α,β-dicarbonyls may also combine reversibly with reduced glutathione to form a hemithioacetal which can then be reduced by aldose reductases, forming lactaldehyde and glycoaldehyde from methylglyoxal and glyoxal, respectively. Alternatively, the hemithioacetal may undergo a two-step conversion to an α-hydroxyacid, catalysed by glyoxalase I and glyoxalase II. The glyoxalase enzyme system represents the major pathway by which methylglyoxal is removed.