Inhibition of Spinach Leaf NADPH(NADH)-Glyoxylate Reductase by Acetohydroxamate, Aminooxyacetate, and Glycidate

Abstract

Glycolate formation during photosynthesis is characteristic of organisms which fix CO2 via RuBP3 carboxylase/oxygenase, particularly when CO2 concentration in chloroplasts is limiting (25,31). An oxygenolytic cleavage of RuBP, catalyzed by the oxygenase activity, followed by hydrolysis of phosphoglycolate by phosphoglycolate phosphatase are considered to be the major, if not the only, reactions leading to glycolate formation in leaves (6,22,25). Spinach chloroplasts also contain low activities (3-6 itmol/h-mg Chl [19,20]) of an NADPH-dependent glyoxylate reductase (GR) capable of producing glycolate from glyoxylate.

Identification of Hydroxypyruvate and Glyoxylate Reductases in Maize Leaves

“…As already mentioned, its affinity for glyoxylate (Km of about 1 mm, Table II) was at least 10-fold lower than that determined for the enzyme from C3 species (17,19). When acetohydroxamate, a strong inhibitor of spinach or pea NADPH(NADH)-GR (1 1,17,20), was included in assays of maize GR, it had virtually no effect at 1 mm concentration and caused only 8 to 10% inhibition at 5 mm (data not shown). By comparison, the same levels of acetohydroxamate caused 60% (1 mM) to 80% (5 mM) inhibition of the crude ammonium sulfate fractionated (45-60% ppt.)…”

“…The high activity of maize NADPH-HPR/GR from the bundle sheath cells (over 100 ,umol h-' mg Chl'), which is the upper range of values reported for C3 enzymes (17,20), seems surprising for a plant that is supposed to have only limited photorespiratory metabolism (13). Perhaps, the activities involved serve some other function(s), not related to photorespiration, but this is hard to reconcile with the view that most hydroxypyruvate and glyoxylate in photorespiring cells is derived from glycolate (13).…”

“…Recently two new leaf enzymes, NADPH-preferring glyoxylate reductase (NADPH[NADH]-GR) and HPR-2, have been characterized, which metabolize glyoxylate and hydroxypyruvate to glycolate and glycerate, respectively, at rates comparable to, or exceeding, an overall rate of carbon flow through the glycolate pathway (17)(18)(19)(20). HPR-2 is localized largely in the cytosol ( 16) and, in analogy to the peroxisomal reductase, shows nonspecific rates with glyoxylate (18).…”

“…HPR-2 is localized largely in the cytosol ( 16) and, in analogy to the peroxisomal reductase, shows nonspecific rates with glyoxylate (18). NADPH(NADH)-GR is also mostly cytosol based (11,16), it is specific for glyoxylate (19), and should be considered as an important glycolate-generating reaction in leaves (17,19,20). Both enzymes have been proposed to serve as back-up reactions of the glycolate pathway, utilizing glyoxylate and hydroxypyruvate generated in the peroxisomes and 'leaked' or otherwise exported to the cytosol (1 1, [16][17][18][19][20].…”

“…NADPH(NADH)-GR is also mostly cytosol based (11,16), it is specific for glyoxylate (19), and should be considered as an important glycolate-generating reaction in leaves (17,19,20). Both enzymes have been proposed to serve as back-up reactions of the glycolate pathway, utilizing glyoxylate and hydroxypyruvate generated in the peroxisomes and 'leaked' or otherwise exported to the cytosol (1 1, [16][17][18][19][20]. The GR recycles glyoxylate back to the glycolate level, the latter is probably transported to the peroxisomes and converted to glycine, while glycerate formed by the cytosolic HPR may be directly exported to the chloroplasts to be metabolized to 3-phosphoglycerate by glycerate kinase (13).…”