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Some Effects of Calcium Ions on the Structure of Bovine Pancreatic Deoxyribonuclease A

From the ¹ From the Department of Biology, University of California, San Diego, La Jolla, California 92037

The effects of divalent metal ions on the circular dichroism (CD) and optical rotatory dispersion (ORD) spectra of bovine pancreatic deoxyribonuclease have been studied. Both the CD and ORD spectra of DNase undergo significant changes below 250 nm upon the binding of calcium to the enzyme. The maximum increase in ellipticity at the 215 nm minimum in the CD spectrum is 950 deg cm² per decimole. Fifty per cent of the maximum transition occurs near 10^-5 m CaCl₂. MnCl₂ gives a smaller increase at 215 nm of 290 deg cm² per decimole while MgCl₂ causes no change.

Calcium is able to protect DNase completely against inactivation by trypsin. Half-maximum protection is achieved near 1.3 x 10^-5 m CaCl₂. Although some protection against trypsin inactivation is given by MnCl₂, and less by MgCl₂, at no concentration is either metal able to protect completely. Calcium-induced changes in the CD spectrum and in susceptibility to trypsin must not involve any large changes in shape or volume of the protein since no change in the sedimentation coefficient could be observed.

Concentrations of 10^-4 m to 10^-2 m CaCl₂ cause intensification of the CD bands between 310 nm and 250 nm. The same calcium concentration range induces a large ultraviolet difference spectrum in DNase. This indicates that calcium causes significant perturbations of tryptophan and tyrosine residues in DNase.

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