Glutamine synthetase from mesophyll and bundle sheath maize cells: isoenzyme complements and different sensitivities to phosphinothricin

Anion-exchange I;PLC has been used to resolve the isoforms of glutamine syn thetase (GS, EC 6.3.1.2) from Zea mays mesophyll (MC) and bundle sheath cel ls (BSC). Two different isoforms were detected in both types of photosynthe tic cells. The predominantly active isoform was GSI (61%) in MC and GS2 (67 %) in BSC. The relative contribution of GS1 and GS2 to the overall GS activ ity in BSC in maize here reported resembles the proportion described for mo st C3 plants. Differences among these isoforms in terms of their susceptibi lity to phosphinothricin (PPT), an analogue of glutamate and known inhibito r of GS, were found. The GS1 isoenzyme from MC was the most sensitive form, being inhibited by 50% at approximately 2.0 muM DL-PPT, whereas the GS2 fr om BSC presented the highest tolerance to the inhibitor (I-50 = 30 muM) The transferase-to-semibiosynthetic activity ratio for the MC isoforms, which was higher than the ratio for the BSC isoforms, and the differences shown b y the isoforms in susceptibility to PPT predict important differences in th e biochemical properties and regulation of GS isoenzymes. In this regard, t he cytoplasmic isoenzymes, and especially the one in MC, due to its relativ ely high contribution to mesophyll cell GS activity, could play a vital rol e in nitrogen metabolism in maize.