Soybean xanthine oxidase

Inhibition and stimulation of LOX activity occurs as a rule by a free radical mechanism. Riendeau et al. [8] showed that hydroperoxide activation of 5-LOX is product-specific and can be stimulated by 5-HPETE and hydrogen peroxide. NADPH, FAD, Fe2+ ions, and Fe3+(EDTA) complex markedly increased the formation of oxidized products while NADH and 5-HETE were inhibitory. Jones et al. [9] also demonstrated that another hydroperoxide 13(5)-hydroperoxy-9,ll( , Z)-octadecadienoic acid (13-HPOD) (formed by the oxidation of linoleic acid by soybean LOX) activated the inactive ferrous form of the enzyme. These authors suggested that 13-HPOD attached to LOX and affected its activation through the formation of a protein radical. Werz et al. [10] showed that reactive oxygen species produced by xanthine oxidase, granulocytes, or mitochondria activated 5-LOX in the Epstein Barr virus-transformed B-lymphocytes. 

Xanthine oxidase activity was detected mainly in the nodules of soybean plants, and showed little activity in the roots of a non-nodulated variety (Tajima et al., 1977). High levels of uricase have been detected in soybean (Tajima et al., 1977) and cowpea (Herridge et al., 1978) nodules. On maturation the enzyme disappeared in the roots but significant activity could be detected in the pod and leaves. Uricase activity has been detected in glyoxy-somes (Theimer and Beevers, 1971) and peroxisomes (Tolbert, 1971). In nodules the enzyme is apparently associated with the bacteroid (Tajimaef al., 1977), although no attempt was made to identify plastids known to be present in root tissue. 

Enzymes ofpurine oxidation High levels of xanthine oxidase, uricase, and allantoinase were reported in soybean nodules by Tajima and Yamamoto (1975). The levels of several of these enzymes were elevated in soybean nodules when compared to lupin (Reynolds et al, 1982a) or pea (Christensen and Jochimsen, 1983). Subsequent work has demonstrated that xanthine oxidase (which is in fact an NAD -dependent dehydrogenase), uricase, and allantoinase activities in the nodules increase dramatically during the onset of N2 fixation and NH4" assimilation in soybean (Schubert, 1981 Reynolds et al, 1982b) and cowpea (Atkins ct fl/., 1980 1984b). 

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