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L-Ornithine:2-Oxoacid Aminotransferase from Squash (Cucurbita pepo, L.) Cotyledons

Purification and Properties ^1,2

^a Department of Food Science and Technology, University of California, Davis, California 95616

Ornithine: 2-oxoacid aminotransferase (EC 2.6.1.13) has been purified over 400-fold with a total recovery of 14% from acetone powders of cotyledons of germinating squash (Cucurbita pepo, L.) seedlings. The pH optimum of the transamination between L-ornithine and -ketoglutarate is 8 and the Michaelis constants are 4.7 mM and 6.3 mM, respectively. The enzyme has a molecular weight of 48,000 as determined by gel filtration. The reaction is essentially specific for -ketoglutarate as the amino group acceptor. The enzyme is inhibited very strongly by hydroxylamine, and less severely by NaCN and isonicotinylhydrazide. No inhibition is observed in the presence of 10 mML-cysteine. The energy of activation is 7.6 kcal/mole. The stability of the enzyme preparation is enhanced by the presence of dithioerythritol and glycerol. The enzyme activity of the most purified fraction is stimulated 30% by the addition of pyridoxal phosphate; however, the evidence for the unequivocal involvement of pyridoxal phosphate was inconclusive.

⁴ To whom requests for reprints should be directed.

¹ This research was supported in part by Contract 2-6633 from the National Aeronautics and Space Administration and Grant AP00699 from the Air Pollution Control Office.

² These studies form part of a dissertation submitted by T. S. L. to the University of California, Davis, in partial fulfillment of the work requirements for the Ph.D. degree. Portions of this work have been presented at the meeting of the American Society of Plant Physiologists at Asilomar, Calif., in August 1971 and that of the American Society of Biological Chemists in Atlantic City, N. J., April 1973.