A conformation change of the porcine intestinal calcium binding protein on binding of Ca2+.

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Abstract
The intrinsic Tyr fluorescence of the porcine intestinal Ca binding protein (CaBP, 7 .mu.M) was quenched by the addition of .apprx. 170 .mu.M ethyleneglycol-bis(2-amino ethylether)-N,N,N''N''-tetraacetic acid (EGTA), returning progressively to its original level with increasing concentration of subsequently added Ca2+ up to 117 .mu.M, in a concentration-dependent manner. In the presence of an excess of EGTA, the intrinsic fluorescence of the CaBP was further quenched by 1 M or less of guanidine hydrochloride, while it was enhanced by 2-4 M guanidine. In the presence of an excess of Ca2+, the fluorescence intensity increased monotonically with increasing concentration of guanidine (.apprx. 4M). Quenching of the intrinsic fluorescence of the CABP by alkaline pH''s (above 8) was moderated by addition of EGTA compared to that measured in the presence of Ca2+. KCL (.apprx. 100 mM) showed a quenching effect on the fluorescence in the presence of 83 .mu.M EGTA, an enhancing effect in the presence of 1 mM EGTA, and no effect in the presence of Ca2+ at a concentration sufficient to saturate the CaBP. Ca2+ binding to the CaBP apparently induces microenvironmental and also significant conformational changes in the Tyr-containing region of the protein.