  In de Sitter ambient space formalism, the massless minimally coupled scalar
field can be constructed from a massless conformally coupled scalar field and a
constant five-vector $A^{\alpha}$. Also, a constant five-vector $B^{\alpha}$
appears in the interaction Lagrangian of massless minimally coupled scalar and
spinor fields in this formalism. These constant five-vector fields can be fixed
in the interaction case in the null curvature limit. Here we will calculate the
$\cal S$ matrix elements of scalar-spinor fields interaction in the tree level
approximation. Then the constant five-vectors $A^{\alpha}$ and $B^{\alpha}$,
will be fixed by comparing the $\cal S$ matrix elements in the null curvature
limits with the Minkowskian counterparts.
