Localization of xanthine dehydrogenase in cowpea root nodules: implications for the interaction between cellular compartments during ureide biogenesis.

Immunocytochemistry was used to assess the location of xanthine dehydrogenase (EC 1.1.1.204) in the infected region of nodules of cowpea (Vigna unguiculata [L.] Walpers cv. Queen Anne Blackeye). Polyclonal antibodies raised against purified cowpea xanthine dehydrogenase were used to localize this enzyme at the electron microscopic level. Sparse nonspecific labeling was observed after treatment of nodule sections with preimmune serum. Although immune serum cross-reacted with the ground cytoplasm of both infected and uninfected cells, significantly more labeling was observed in the uninfected cells. No labeling above background was observed in peroxisomes, mitochondria, proplastids, endoplasmic reticulum, cytoplasmic or peribacteroid membranes, peribacteroid spaces, or bacteroids. The enzyme is soluble and not present in any organelle or membrane. The greater concentration of xanthine dehydrogenase in the uninfected cells suggests that xanthine or a precursor to xanthine, rather than uric acid, is the intermediate that moves from infected to uninfected cells during ureide biogenesis.