Enzymes of purine catabolism in soybean plants

Remarkable formation and utilization of allantoin is observed in soybean (Glycine max variety A62-1). To study this, various enzymes involved in purine catabolism (i.e., xanthine oxidase, uricase and allantoinase) were measured in different regions of soybean plants during development. Uricase, which catalyzes the direct formation of allantoin from uric acid, was studied in detail. The activities of these three enzymes were highest in the root nodules, indicating that the nodules are the major site of allantoin metabolism. Radicles only showed appreciable activity about 80 hr after the seeds were planted. Allantoinase activity was detected in all regions tested, showing that allantoin translocated from the nodules can be metabolized in the roots, stem and leaves. In the nodules, xanthine oxidase was localized in the nuclear fraction, while uricase was mainly restricted to the mitochondrial fraction and allantoinase to the soluble fraction. Uricase was partially purified from the nodules and radicles, respectively. The pH optimum of enzyme from the nodules was 9.5, whereas that of enzyme from the radicles was 7.0. The enzyme from the nodules did not require a cofactor, while that from the radicles showed an absolute requirement for a cofactor, which was a low molecular substance easily separable from the apoprotein. Thus, the uricase in nodules differs in chemical properties from that in the host plant. The results are discussed in relation to change in the allantoin level in soybean tissues.