Aspartate and alanine movement across symbiotic membranes of soybean nodules

Aspartate, alanine and glutamate uptake into bacteroids and symbiosomes isolated from soybean nodules were measured. An aspartate transport mechanism was identified in free bacteroids, with an apparent K(m) of 27.5 μM and a V(max) of 5.14 nmol mg^-1 min^-1. Inhibitor studies indicated that aspartate uptake was dependent on a proton motive force across the bacteroid membrane. Aspartate transport was competitively inhibited by glutamate with an apparent K(i) of 9.7 μM. Alanine uptake into bacteroids also showed saturation kinetics with a K(m) of 12.2 μM and V(max) of 3.5 nmol mg^-1 min^-1. Bacteroids loaded with ¹⁴Caspartate exchanged label for external cold aspartate and glutamate. In contrast, there was no evidence for aspartate or alanine movement into or out of intact symbiosomes except by slow passive diffusion.