Asparagine synthetase in Zea mays

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Abstract
Properties of asparagine synthetase from various tissues of Z. mays L. (embryo and endosperm from developing seeds; scutella and roots from young seedlings) and from cotyledons of Glycine max L. were compared. The specific activities obtained with extracts from soybean cotyledons were 10- to 30-fold higher than activities obtained from any of the maize tissues. The apparent Km values (mM) were about 0.5 for glutamine and about 2.0 for NH4+ for the enzymes from the various corn tissues and 0.18 and 3.0, respectively, for soybean cotyledons. The Vmax values were about 2-fold higher for glutamine compared with NH4+ for corn embryos, endosperms and scutella and for soybean cotyledons. With corn roots, on the other hand, a slightly higher Vmax was obtained when NH4+ was the N donor. With the exception of corn root enzyme, glutamine protected asparagine synthetase from heat inactivation. NH4+ had no protective effect on the enzyme from any of the tissues examined. The enzyme from corn (any tissue) is not as stringent in its requirement for glutamine as is the enzyme from soybean cotyledons and the root enzyme responds to NH4+ and glutamine in a unique fashion. The enzyme obtained from developing seeds (embryo or endosperm) was inhibited by asparagine. All the corn enzymes were inhibited by ADP to a greater extent than by AMP or ATP. Enzyme from the scutella or soybean cotyledons but not from the root or the developing embryo was inhibited by the glutamine analog albizziin. Apparently there are important differences in the properties of asparagine synthetases extracted from various tissues of corn and soybean cotyledons.