  Deep inelastic scattering of leptons from hadrons at small values of the
Bjorken parameter $x$ is studied from superstring theory. In particular, we
focus on single-flavored scalar and vector mesons in the large $N$ limit. This
is studied in terms of different holographic dual models with flavor Dp-branes
in type IIA and type IIB superstring theories, in the strong coupling limit of
the corresponding dual gauge theories. We derive the hadronic tensor and the
structure functions for scalar and polarized vector mesons. In particular, for
polarized vector mesons we obtain the eight structure functions at small values
of the Bjorken parameter. The main result is that we obtain new relations of
the Callan-Gross type for several structure functions. These relations have
similarities for all different Dp-brane models that we consider. This would
suggest their universal character, and therefore, it is possible that they hold
for strongly coupled QCD in the large $N$ limit.
