  It is stressed that within large N_c-expansion {\bf analytical calculations}
of the $(\alpha_s/\pi)^2$ QCD contributions to the valence part of the
Gottfried sum rule for F_2 structure function of charged leptons-nucleon
deep-inelastic scattering reveals the existence of the $O(1/N_c^2)$ {\bf
non-planar} corrections only and the disappearance of the {\bf planar} O(N_c^0)
perturbative terms. The relation between Gottfried and Adler sum rule for
neutino-nucleon DIS is established and the proposal that the differebce between
corresponding QCD corrections to higher non-singlet moments in charged-lepton
and neutrino DIS are {\bf suppressed by 1/N_c^2} is made. The possible
consequence of the cancellation of {\bf perturbative planar} graphs in the
considered perturbative series, namely the existence of light-quark flavour
asymmetry $\bar{u}(x)<\bar{d}(x)$, is mentioned. The effect of the similar
origin, i.e. the relation of {\bf ``light-by-light-type''-type structure} in
DIS characteristics at the $\alpha_s^3$-level to the generation of light
quark-antiquark asymmetry, is also commented.
