Monomer-dimer equilibria of a Bence Jones protein and its variable fragment

The circular dichroic (CD) spectra of a type lambda Bence Jones protein (Tod), its variable (VL) fragment, and the constant (CL) fragment of a type lambda protein (Nag) were measured under various conditions. In the pH region from 5.5 to 7.5, the CD spectra of Tod protein with intact interchain disulfide bond (L(SS)) and and CL did not change with pH, while the spectra of Tod protein in which the interchain disulfide bond had been reduced and alkylated (L(RA)) and VL did not change with pH. The dimerization reactions of L(RA) and VL were studied by following the CD change with protein concentration. The CD spectrum of CL did not change with the protein concentration. The dimerization constant for L(RA) was 4 X 10(4) M-1 at at pH 7.5 and 25 degrees C, which was smaller than that for VL (1 X 10(5) M-1). The ellipticity at 278 nm for the L(RA) dimer was different from that for the L(SS) dimer and changed with pH. These findings indicate that the L(RA) dimer and L(SS) dimer have different conformations. The differences in the conformation and L-L interaction between the L(RA) dimer and L(SS) dimer are discussed on the basis of the conformations of VL and CL and the interactions between the paired domains.