The apparent absorbance of a globular metalloprotein molecule due to Rayleigh scattering may be comparable to the absorbance associated with ligand field transitions of the metal ion. The apparent molar scattering coefficient ετ for protein subunits may be calculated from the approximate expression ετsubunit = (32π3n02(dn/dX)2/3000(ln 10)N)·(M2/qλ4) where n0 is the refractive index of the solvent, dn/dX is the specific refractive index increment of the protein, X is the concentration of the protein in g·cm-3, M is the molecular weight of the protein, N is Avogadro's number, q is the number of identical subunits and λ is the wavelength of light (in vaccum) in cm. For a typical globular protein, ετ = 1.45·10-26 M2·q-1·λ-4 at 25°C. After the spectrum of the large nickel-containing enzyme, urease (urea amidohydrolase, EC 184.108.40.206), has been corrected for scattering, the absorption peaks associated with nickel ion are fully consistent with Ni(II) ion in an octahedral complex. When the competitive inhibitor, β-mercaptoethanol, binds to urease, new absorption peaks appear at 324, 380 and 420 nm (shoulder) with molar absorption coefficients of 1550, 890 and 460 M-1·cm-1, respectively. These peaks may be assigned to RS- → Ni(II) charge transfer transitions by comparison with spectra of model complexes. The results establish that β-mercaptoethanol coordinates directly to active-site Ni(II) ion in urease. © 1983.