Role of Inosine Monophosphate Oxidoreductase in the Formation of Ureides in Nitrogen-Fixing Nodules of Cowpea (Vigna unguiculata L. Walp.)

Publisher Website
Google Scholar
Abstract
Cell-free extracts from nodules of cowpea (V. unguiculata L. (Walp.) cv. Caloona:Rhizobium strain CB756) prepared in the presence of 15% (vol/vol) glycerol showed high rates (30-60 nmoles NAD reduced/min per g fresh weight nodule) of IMP oxidoreductase (EC 1.2.1.14) activity. The enzyme was labile (half-life of activity less than 3 h) but could be stabilized for up to 18 h by inclusion of the substrates NAD and IMP in the breaking media. Activity showed a broad pH optimum between 8.5 and 9.5, had an apparent Km (IMP) of 4 and 12 .mu.M at pH 7.5 and 9.0, respectively, and was largely (96%) associated with the plant cell cytosol fraction of the nodule. Metabolism of [8-14C]IMP and [1-14C]glycine by the cell-free system showed 2 pathways for purine base production from IMP, one via xanthosine monophophate, xanthosine and xanthine, the other via inosine and hypoxanthine. The proportion of IMP utilized by inosine monophosphate oxidoreductase and the xanthine-based pathway was increased from 30% at 0.5 mM to 80% at 0.01 mM IMP. In vivo, inosine monophosphate oxidation, rather than dephoshorylation, appears to be the predominant metabolic route leading to ureide synthesis, and IMP provides the link between de novo purine nucleotide synthesis in the plastid and ureide production in the plant cell cytosol.