Subcellular location of enzymes of ammonia assimilation and asparagine synthesis in root nodules of Lupinus albus L.

Abstract

The subcellular localization of enzymes involved in ammonia assimilation and synthesis of asparagine in the host cells of the central, N₂-fixing zone of nodules of Lupinus albus L. has been examined using discontinuous and continuous sucrose density gradient separation of the soluble, plant organelle and bacteroid components of nodule extracts. Both glutamine synthetase (EC 6.3.1.2) and asparagine synthetase (EC 6.3.5.4) activities were cytosolic while significant NADH: glutamate synthase (EC 2.6.1.53) and aspartate aminotransferase (EC 2.6.1.1) activities were recovered together with starch-containing plastids. NAD: glutamate oxidoreductase (glutamate dehydrogenase; EC 1.4.1.2) was associated almost completely with intact mitochondria. The distribution of enzymes indicates complex subcellular organization of pathways of asparagine synthesis.