  The non-photonic neutrino exchange mechanism of the lepton flavor violating
muon-electron conversion in nuclei is revisited. First we determine the nucleon
coupling constants for the neutrino exchange mechanism in a relativistic quark
model taking into account quark confinement and chiral symmetry requirements.
This includes a new, previously overlooked tree-level contribution from
neutrino exchange between two quarks in the same nucleon. Then for the case of
an additional sterile neutrino we reconsider the coherent mode of this process.
The presence of a mixed sterile-active neutrino state heavier than the quark
confinement scale Lambda_c (~1 GeV) may significantly improve the prospects for
observation of this process in future experiments as compared to the
conventional scenario with only light neutrinos. Turning the arguments around
we derive new experimental constraints on |U_(mu h)| and |U_(e h)| mixing
matrix elements from the non-observation of muon--electron conversion.
