Glutamine Synthetase of Germinating Peanuts

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Abstract
Glutamine synthetase activity, extracted from an acetone powder of 7-day germinated peanuts (Arachis hypogaea L.), was precipitated by ammonium sulfate (40-60% saturation) and further purified by gel filtration and calcium phosphate gel treatment. When it was adsorbed to and subsequently eluted from a column of diethylaminoethyl-cellulose, two peaks of activity (designated glutamine synthetase 1 and 2) were obtained which were enriched 150- and 20-fold, respectively, over the initial extract. Glutamine synthetase 1 was present in ungerminated seeds and in the cotyledons during germination; glutamine synthetase 2 appeared during germination and was found largely in the developing plant. Compared with glutamine synthetase 2, glutamine synthetase 1 appeared to have a slightly smaller molecular weight and was more stable to heat and storage. The catalytic properties of the two forms were essentially the same. Whereas neither form catalyzed γ-glutamyltransferase activity with 4-methyleneglutamine, both glutamine synthetases 1 and 2 catalyzed an ATP- and NH4+-dependent conversion of [14C]-4-methyleneglutamic acid to [14C]-4-methyleneglutamine, but the Km value for 4-methyleneglutamic acid was 10-fold greater and the Vmax only one-fourth that measured with l-glutamic acid. This is the first report of glutamine synthetase activity with 4-methyleneglutamic acid as substrate, although the level of this activity does not appear adequate to account for the rapid synthesis of 4-methyleneglutamine observed in germinating peanuts.