Proton Stability in Leptoquark Models
  We show that in generic leptoquark (LQ) extensions of the standard model lepton and baryon numbers are broken at the level of renormalizable operators. In particular, this may cause fast proton decay unless the leptoquarks are heavy enough. We derive stringent bounds for the 1st generation LQ masses and couplings from the proton stability constraints. 
Sergey Kovalenko and Ivan Schmidt