Carbon Dioxide Fixation by Lupin Root Nodules

Abstract

Labeling studies using detached lupin (Lupinus angustifolius) nodules showed that over times of less than 3 min, label from [3,4-14C]glucose was incorporated into amino acids, predominantly aspartic acid, to a much greater extent than into organic acids. Only a slight preferential incorporation was observed with [1-14C]- and [6-14C]glucose, while with [U-14C]-glucose more label was incorporated into organic acids than into amino acids at all labeling times. These results are consistent with a scheme whereby the carbon skeletons for amino acid synthesis are provided by the phosphoenolpyruvate carboxylase [EC 4.1.1.31] reaction. A comparison of 14CO2 release from nodules supplied with [1-14C]- and [6-14C]glucose indicated that the oxidative pentose phosphate pathway accounted for less than 6% of glucose metabolism. Several enzymes of the oxidative pentose phosphate and glycolytic pathways were assayed in vitro using the 12,000 g supernatant fraction from nodule homogenates. In all cases, the specific activities were adequate to account for the calculated in vivo fluxes. Three out of 4 diverse treatments that inhibited nodule N fixation also inhibited nodule CO2 fixation, and in the case of the 4th treatment, replacement of N2 with He, it was shown that the normal entry of label from exogenous 14CO2 into the nodule amino acid pool was strongly inhibited.