  The Dirac equation offers a precise analytical description of relativistic
two-particle bound states, when one of the constituent is very heavy and
radiative corrections are neglected. Looking at the high-Z hydrogen-like atom
in the infinite momentum frame and treating the electron as a "parton", various
properties usually attributed to the quark distributions in the nucleon are
tested, in particular: Bj{\o}rken scaling; charge, helicity, transversity and
momentum sum rules; existence of the parton sea; Soffer inequality; correlation
between spin and transverse momentum (Sivers and Boer-Mulders effects);
transverse displacement of the center-of-charge and its connection with the
magnetic moment. Deep inelastic experiments with photon or positron beams at
MeV energies, analogous to DIS or Drell-Yan reactions, are considered.
