Oncomodulin and parvalbumin. A comparison of their interactions with europium ion.

The 7F0----5D0 transition of Eu3+ was used to probe the metal-binding domains of rat oncomodulin and rat parvalbumin. Two distinct differences between the two proteins were observed. The first relates to the pH-dependent behavior of their 7F0----5D0 spectra, a phenomenon noted previously for other paravalbumins. In the case of rat parvalbumin, the spectral features associated with both metal-binding sites titrate concomitantly (pK alpha = 8.2); however, in the case of oncomodulin, the two sites titrate sequentially (pK alpha = 6.3 for the CD site; pK alpha = 8.3 for EF site). The proteins also contrast with regard to their discrimination for Eu3+ over Ca2+. The CD and EF sites in rat parvalbumin both display a large preference for Eu3+: (KCa/KEu)CD = 143 +/- 11 and (KCa/KEu)EF = 191 +/- 30. However, in the case of oncomodulin, although the EF site of oncomodulin greatly prefers the trivalent lanthanide ion (KCa/KEu = 300 +/- 80), the CD site exhibits a relatively minor preference (KCa/KEu = 11 +/- 1).