  The processes of charge-exchanges of $\pi^{-}$ and $K^{-}$ on protons at high
energies and zero transfers are described on the basis of the parton model. It
is shown that at the mentioned kinematic conditions the scattering occurs on
the valence quarks only, while scattering on sea quarks leads to disruption of
final state. Thereby a substantiation is given of the valence approximation for
the specified reaction. On the basis of GAMS-4$\pi$ data it is shown that the
description of $\eta - \eta'$ mixing does not require an introduction of
additional states that do not fit into the scheme of basis states $|\eta_N >$
and $|\eta_S >$. The resulting value of the mixing angle is $\phi_p=(36.1\pm
0.9)^0$.
