Glutamate dehydrogenase higher plants

Glutamic acid dehydrogenase is widely distributed in microorganisms and higher plants as a catalyst in the synthesis of L-glutamic acid from a-ketoglutaric acid and free ammonia. Transaminase is contained in a wide variety of microorganisms. 

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. 

The final step in the conversion of proline to glutamate is the oxidation of pyrroline-5-carboxylate to glutamate. Since pyrroline-5-carboxylate is in equilibrium in solution with its straight chain form, glutamate-y-semialdehyde it is possible that the latter compound is the substrate for this last step. An enzyme A -pyrroline-5-carboxylate dehydrogenase (E.C. 1.5.1.12) has been found in mitochondrial preparations fixim several higher plant seedlings (Stewart and Lai, 1974) which will catalyze this reaction. 

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 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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