Urea Is a Product of Ureidoglycolate Degradation in Chickpea. Purification and Characterization of the Ureidoglycolate Urea-Lyase1

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American Society of Plant Physiologists

RESUMO

A ureidoglycolate-degrading activity was analyzed in different organs of chickpea (Cicer arietinum). Activity was detected in all the tissues analyzed, but highest levels of specific activity were found in pods, from which it has been purified and characterized. This is the first ureidoglycolate-degrading activity that has been purified to homogeneity from any photosynthetic organism. Only one ureidoglycolate-degrading activity was found during the purification. The enzyme was purified 1,500-fold, and specific activity for the pure enzyme was 8.6 units mg−1, which corresponds with a turnover number of 1,600 min−1. The native enzyme has a molecular mass of 180 kD and consists of six identical or similar-sized subunits of 31 kD each. The enzyme exhibited hyperbolic, Michaelian kinetics for (−) ureidoglycolate with Km values of 6 and 10 μm in the presence or absence of Mn2+, respectively. Optimum pH was between 7 and 8 and maximum activity was found at temperatures above 70°C, the enzyme being extremely stable and resistant to heat denaturation. The activity was inhibited by EDTA and enhanced by several bivalent cations, thus suggesting that the enzyme is a metalloprotein. This enzyme has been characterized as a ureidoglycolate urea-lyase (EC 4.3.2.3), which catalyzes the degradation of (−) ureidoglycolate to glyoxylate and urea. This is the first time that such an activity is detected in plant tissues. A possible function for this activity and its implications in the context of nitrogen mobilization in legume plants is also discussed.

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