Equilibrium binding constants of C3DP, a nucleic acid-binding protein isolated from human serum.

The malignancy-associated DNA-binding protein C3DP has been isolated from normal human serum and identified by dodecyl sulfate/polyacrylamide gradient gel electrophoresis and immunodiffusion. The binding of purified C3DP to DNA and RNA has been investigated by density gradient centrifugation. The equilibrium binding constant K of C3DP binding to native human DNA was determined to be K = (2.9 +/- 0.5) x 10(5) M-1. From this value we conclude that C3DP binds non-specifically with regard to the base sequence of the DNA. K was found to be in the same order of magnitude when measured with single-stranded or double-stranded DNA of varying G + C-content, and with RNA. It varied only slightly with the pH or ionic strength of the medium, indicating that only one Na ion is released from DNA when one molecule of C3DP is bound. We conclude that ionic interactions cannot account for the stability of the C3DP-DNA complex and that therefore hydrogen bonds and hydrophobic interactions are likely to be formed.