To explore the possibility that the affinity of some myeloma proteins for 2,4-dinitrophenyl (DNP) ligands is the consequence of a "strange" (i.e., unexpected) cross-reaction for more natural ligands, a variety of substances (primarily derivatives of purines, pyrimidines, naphthaquinone) were tested for ability to block the binding of [3H]-ϵ-DNP-L-lysine by protein 315, an IgA mouse myeloma protein with high affinity for DNP ligands. The most impressive inhibiting activity was observed with 2-methyl-1,4-napthaquinone (menadione, vitamin K3). The affinity (intrinsic association constant) of protein 315 for menadione was 5 x 105 L/M (at 4°C). Because the same affinity was measured in direct-binding assays (e.g., equilibrium dialysis) and in an indirect one based on the assumption of competitive binding with DNP-lysine, it is likely that menadione and DNP bind at overlapping sites in the protein's combining region. This conclusion is supported by molecular models which reveal some common structural features in these ligands. Hence it is not surprising that antinitrophenyl antibody preparations, raised by conventional immunization procedures (anti-2,4-DNP; anti-2,6-DNP; anti-2,4,6-TNP) also bind menadione with considerable affinity. As with DNP ligands, when menadione binds to protein 315 or to conventional antinitrophenyl antibodies, some of the protein's tryptophan fluorescence is quenched, there is a change in the ligand's absorption spectrum (hypochromia and/or red shift), and the binding is temperature-dependent (exothermal).

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