Jack bean urease (EC 3.5.1.5). I. A simple dry ashing procedure for the microdetermination of trace metals in proteins. The nickel content of urease

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Abstract
A simple and inexpensive procedure for determination of microgram quantities of metal ions in proteins is described and tested with Ni and Fe. The method involves dry ashing in an oxygen atmosphere at 450-460.degree. C in Pyrex vessels, conversion of the metal oxides or other compounds to readily soluble species, and spectrophotometric analysis. An improved procedure for the direct spectrophotometric determination of Ni using dimethylglyoxime is accurate to .+-. 2% or better with samples of 1-5 .mu.g of Ni. These techniques were used to determine that the Ni content of freshly prepared jack bean urease is 2.00 .+-. 0.12 g-at./96,600 g protein. This corresponds to 2.0 Ni ions per subunit. This result was confirmed by atomic absorption analysis, which also showed that Ca, Mn, Co and Fe are not present in significant amounts in urease.