  We compute the distribution amplitudes of the pion and kaon in the
light-front constituent quark model with the symmetric quark-bound state vertex
function. In the calculation we explicitly include the flavor-SU(3) symmetry
breaking effect in terms of the constituent quark masses of the up (down) and
strange quarks. To calculate the kaon parton distribution functions~(PDFs), we
use both the conditions in the light-cone wave function, i.e., when $\bar{s}$
quark is on-shell, and when $u$ quark is on-shell, and make a comparison
between them. The kaon PDFs calculated in the two different conditions clearly
show asymmetric behaviour due to the flavor SU(3)-symmetry breaking implemented
by the quark masses.
