  The recently observed multiferroic behavior in the s=1/2 chain cuprate
LiVCuO_4 (LiCuVO_4) with edge-shared CuO_4 plaquettes and helical spin ordering
does not agree with the existing theories such as a spin-current scenario. We
argue that the effect can be consistently explained, if the exchange-induced
electric polarization on the out-of-chain Cu^2+ centers substituting for
Li-ions in LiVCuO_4 is taken into account. These substituent centers are proved
to be an effective probe of the spin incommensurability and the magnetic field
effects.
