THE ENZYMATIC REDUCTION OF GLYOXYLATE AND HYDROXYPYRUVATE IN LEAVES OF HIGHER-PLANTS

Glyoxylate and hydroxypyruvate are metabolites involved in the pathway of carbon in photorespiration. The chief glyoxylate-reducing enzyme in leaves is now known to be a cytosolic glyoxylate reductase that uses NADPH as the preferred cofactor but can also use NADH. Glyoxylate reductase has been isolated from spinach leaves, purified to homogeneity, and characterized kinetically and structurally. Chloroplasts contain lower levels of glyoxylate reductase activity supported by both NADPH and NADH, but it is not yet known whether a single chloroplastic enzyme catalyzes glyoxylate reduction with both cofactors. The major hydroxypyruvate reductase activity of leaves has long been known to be a highly active enzyme located in peroxisomes; it uses NADH as the preferred cofactor. To a lesser extent, NADPH can also be used by the peroxisomal enzyme. A second hydroxypyruvate reductase enzyme is located in the cytosol; it preferentially uses NADPH but can also use NADH as cofactor. In a barley mutant deficient in peroxisomal hydroxypyruvate reductase, the NADPH-preferring cytosolic form of the enzyme permits sufficient rates of hydroxypyruvate reduction to support continued substrate flow through the terminal stages of the photosynthetic carbon oxidation (glycolate/glycerate) pathway. The properties and metabolic significance of the cytosolic and organelle-localized glyoxylate and hydroxypyruvate reductase enzymes are discussed.