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Inactivation of Serine:Glyoxylate and Glutamate:Glyoxylate Aminotransferases from Tobacco Leaves by Glyoxylate in the Presence of Ammonium Ion

Department of Biochemistry and Genetics, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504

Serine:glyoxylate and glutamate:glyoxylate aminotransferases (SGAT and GGAT), which catalyze the formation of glycine from glyoxylate during photorespiration, have been purified >300-fold from tobacco leaf extracts. Incubation with glyoxylate in the absence of amino acid substrate resulted in the time- and concentration-dependent inhibition of the partially purified enzymes. The second order rate constants for glyoxylate inhibition were 1.25 and 0.175 per millimolar per minute for SGAT and GGAT, respectively. The enzymes of highest specific activity were not inhibited by 5 millimolar glyoxylate alone but when 1 millimolar NH₄⁺ was added to 1 millimolar glyoxylate for 10 min in the absence of amino donor, SGAT was irreversibly inhibited more than 50%. GGAT was inhibited 50% in 10 minutes by 15 millimolar NH₄⁺ and 1 millimolar glyoxylate. By itself, NH₄⁺ was a reversible inhibitor; SGAT and GGAT were inhibited 50% at 6 millimolar and 50 millimolar, respectively. The irreversible inhibition occurred only with glyoxylate and NH₄⁺ added together; oxalate, formate, acetaldehyde, pyruvate, hydroxypyruvate, and -ketoglutarate did not inhibit either in the presence or absence of NH₄⁺. Glyoxylate and NH₄⁺ could form a carbinolamine which is a serine analog and might bind irreversibly to the enzymes. Under conditions in vivo in which reassimilation of NH₄⁺ is reduced or blocked, the activity of SGAT may be inhibited and if glyoxylate is available, as in leaf peroxisomes, irreversible inhibition may occur.