  The conversion of protons to positrons at the horizon of a black hole (BH) is
considered. It is shown that the process may efficiently proceed for BHs with
masses in the range $\sim 10^{18}$ -- $10^{21}$ g. It is argued that the
electric charge of BH acquired by the proton accretion to BH could create
electric field near BH horizon close to the critical Schwinger one. It leads to
efficient electron-positron pair production, when electrons are back captured
by the BH while positrons are emitted into outer space. Annihilation of these
positrons with electrons in the interstellar medium may at least partially
explain the origin of the observed 511 keV line.
