Technical Abstract: Abstract: The nutritional physiology of nickel (Ni), an essential nutrient element, is poorly understood. The recent discovery of Ni deficiency in certain horticultural crops evokes multitudes of questions regarding the role of Ni in metabolism, physiology, and culture of crops. We found for pecan (Carya illinoinensis), a woody perennial), that nitrogen (N) metabolism is greatly influenced by plant Ni status via its influence on ureide catabolism, amino acids metabolism, and mobilization of N reserves. Spring xylem sap differs in ureide composition depending upon Ni status, resulting in accumulation of asparagine anhydrous, but reduction of citrulline, allotonic acid, total ureides and urease activity. Of especial significance is that sap also contained a novel 14 kDa protein possessing urease activity. Its N-terminal amino acids (residues 1 - 17) are identical to 13,473 Da bovine pancreatic RNase A (residues 5 - 21) and its chain A residues (1 - 17). Thus the pure protein is designated "RNase A - Urease" (EC 3.1.27.5 - EC 3.5.1.5) and a previously unknown third urease isoform in plants. RNase A - Urease requires Ni ions for activation and appears to assist in the catabolism of stored N forms being translocated in spring xylem sap to growing points. Enzyme assays also revealed that bovine pancreatic RNase A possesses substantial urease catalyzing activity. Because RNase A is also a urease, the discovery implicates a generally ubiquitous role for RNase A in the cycling of N in plant and animal cells and highlights the importance of Ni nutritional status.