As a cofactor, nickel occurs infrequently. About the only known occurrence of nickel is in microbial and plant enzymes such as urease from jack bean, soy bean, rice, and tomatoes. There are roughly two gram-atoms of nickel per mole of the 96 000 Da subunits of the enzyme. Other metalloenzymes containing nickel include Factor F430 found in the membrane of methanogenic bacteria, carbon monoxide dehydrogenase, and hydrogenases I and II. Nickel has drawn the attention of nutritionists because of the observation that nickel concentrations in the serum of women rise sharply immediately after parturition.

Reactivity Some consider nickel the 'metal that was.' As biosystems evolved and moved from an atmosphere of no oxygen to one rich in oxygen, where methane and H2 have tended to be minimized as energy substrates, metals that formed a major cofactor in the anaerobic environment and were used by the more primitive organisms such as archaebacteria have been replaced in favor of a metal or cofactor more suitable to the present day environment. Thus, nickel, like cobalt, may have had its greatest era in enzymes that catabolized CH4 or H2.

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