Nickel legume

It has not been possible so far to establish that Cr is an essential element required by plants, however, addition of Cr to soils deficient in the element has been shown to increase growth rates and yields of potatoes, maize, rye, wheat or oats (Scharrer and Schropp, 1935 Huffman and Allaway, 1973 Bertrand and De Wolf, 1986). Nickel appears to be an essential element for plants (Farago and Cole, 1988). Zerner and coworkers (Dixon et al., 1975) demonstrated that urease isolated from jack bean (Canavalia ensiformis) was a nickel enzyme. Eskew et al. (1983) have shown that Ni is an essential micronutrient for legumes. Most plants contain nickel in the range 1 - 6 mg kg-1 (Vanselow, 1966 Hutchinson, 1981). The uptake of Ni is enhanced by low pH values, and available nickel increases at pH less than 6.5 as a consequence of the breakdown of Ni complexes in the soil with Fe and Mn oxides. Uptake of nickel by plants and questions of toxicity and tolerance have been reviewed by Farago and Cole (1988). Nickel toxicity toward plants has been reviewed by Vanselow (1966) and Hutchinson (1981). 

Phytates found in cereals, legumes, nuts and oil seeds form complexes with minerals, the mineral-phytate complexes in decreasing order of stability being zinc > copper > nickel > cobalt > manganese > calcium. Thus, zinc is affected most. An increase in pH results in phytic acid becoming more ionized and initiates binding to cations. 

Generally in humans oral intake of nickel is derived primarily fix)m food [12,13]. Nickel concentrations in food are usually below 0.5 mg/kg fresh weight. Cocoa, soybeans, and some dried legumes, various nuts, and oatmeal contain higher concentrations of nickel [14,15]. 

So far there is no known specific biochemical function for nickel in animal organisms. In plants and microorganisms, some metal-loenzymes containing nickel have been found. Such an enzyme is urease, found in soybeans, other legumes (this enzyme was first isolated from the seeds of the Jack bean, Canavalia ensiformis), rice and tobacco. Molecule of urease (580 kDa) contains 12 nickel atoms in six subunits. In the active enzyme centre, two nickel atoms are coordinated by four histidyl residues and one residue of e-N-carbamoyl lysine, while the carbamoyl group and one molecule of water form a bridge between the two nickel atoms. Urease catalyses the hydrolysis of urea to ammonia and carbamate (carbamic acid), which is hydrolysed spontaneously to hydrogen carbonate (bicarbonate) and ammonium ions ... 

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