Glutamate dehydrogenase plants

Maestri, E., Restivo, F. M., Gulli, M., and Tassi, F., 1991, Glutamate-dehydrogenase regulation in callus-cultures of Nicotiana plumbaginifolia - effect of glucose feeding and carbon source starvation on the isoenzymatic pattern, Plant Cell Environ., 14 613-618. 

Studies of the glutamate dehydrogenases from green plants indicate that they require a very high concentration of NH3 to be effective. In fact an alternative pathway is re-... 

Inokuchi, R., Itagaki, T., Wiskich, J., Nakayama, K., and Okada, M. (1997). An NADP-glutamate dehydrogenase in the green alga Bryopsis maxima. Purification and properties. Plant Cell Physiol. 38, 327-335. 

In higher plants, glutamate dehydrogenase (GDH) activity has been found in most species tested including maize (Bulen, 1956), oats (Barash et al., 1973), wheat (Nicklish et al., 1976), barley (Miflin, 1970), peas (Yamasaki and Suzuki, 1969), broad bean (Thurman, 1%5), lettuce (Lea and Thurman, 1972), and apple tree (Cooper and Hill-Cottingham, 1974). GDH activity has been found in both bacteroid and cytosol fractions of root nodules of a number of legumes (Brown and Dilworth, 1975). NAD-linked GDH activity has also been found in certain species of bryophytes and pteridophytes tested (Lee and Stewart, 1978). 

By far the most information on the structure of GDH enzymes has come from studies on the enzymes from bovine liver and Neurospora crassa. These studies have been extensively reviewed elsewhere (Smith et ai, 1975). An abbreviated discussion will be presented here together with the more limited information available on higher plant glutamate dehydrogenases. 

Fig. 2. The incorporation of ammonia derived by a number of pathways, into asparagine in higher plants, a. Nitrate reductase b, nitrite reductase c, nitrogenase .d, glutamine synthetase e, asparagine synthetase f, arginase g, urease h, ornithine transaminase i, glutamate dehydrogenase j, proline dehydrogenase k, transaminases.

In plants, molds and bacteria, A. a. into the amino group of glutamate is also possible by NADPH-de-pendent glutamate dehydrogenase (EC 1.4.1.3) the enzyme is most effective when ammonium salts are available directly from the environment in relatively... 

Conn, ed.), pp. 249-268. Academic Press, New York 1981 Stenflo, J. Vitamin K, prothrombin and y-carboxyglutamic acid. Adv. Enzymol. 46, 1-31 (1978) Stewart, G. R., Mann, A. F., Fentem, P. A. Enzymes of glutamate formation glutamate dehydrogenase, glutamine synthetase, and glutamate synthase. In The Biochemistry of Plants, Vol. 5, Amino Acids and Derivatives (J. B. Miflin, ed.), pp. 271-327. Academic Press, New York 1980... 

Glutamic dehydrogenase has been detected in bacteria, yeast, plants, and animal tissues. The enzyme has been purified extensively only from liver, and the properties of the ezyme from other sources are not known very precisely. It has been reported that glutamic dehydrogenase of plants requires DPN, while the enzyme of yeast and E. coli requires TPN. The mammalian enzyme uses both coenzymes. 

GS activity from a wide spectrum of mono- and dicotyledonous species is inhibited by PPT, whether or not both isozymes are present very similar kinetic parameters occur in the different plant species studied. - Neither GOG AT nor glutamate dehydrogenase is affected by PPT in uifro. ... 

PPT application results in a 50% decrease in GS levels in leaf tissues glutamate dehydrogenase levels are enhanced but, as intracellular ammonia levels rise, the amination reaction catalyzed by glutamate dehydrogenase insufficiently compensates for the inhibition of GS. Nitrate reductase is reduced, but proteolytic activity is unaffected. A wide-ranging survey of GS levels in plants reported a 70-fold variation in extractable GS levels there was also considerable variation in the ratios of the cytosolic and chloroplastic isozymes. The kinetics of inhibition by PPT, however, were very similar, and there was no apparent correlation between natural GS levels and herbicide susceptibility. PPT uptake and transport therefore seem to be the determining factors in natural herbicide sensitivity. 

In 1980, Miflin and Lea pointed out that much of the information on biochemical pathways has arisen from the use of mutants of bacteria and yeast larking key enzymes. They complained at the time that no mutants of higher plants were available in the ammonia assimilation pathway. A glance at Table I clearly indicates that a number of such mutants are now available. In addition mutants of maize with low (but not zero) levels of glutamate dehydrogenase have also been studied (Rhodes et ai, 1989). 

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