  We obtain a realization of the Lie superalgebra $D(2, 1 ; \alpha)$ in
differential operators on the supercircle $S^{1|2}$ and in $4\times 4$ matrices
over a Weyl algebra. A contraction of $D(2, 1 ; \alpha)$ is isomorphic to the
universal central extension $\hat{\p\s\l}(2|2)$ of $\p\s\l(2|2)$. We realize it
in $4\times 4$ matrices over the associative algebra of pseudodifferential
operators on $S^1$. Correspondingly, there exists a three-parameter family of
irreducible representations of $\hat{\p\s\l}(2|2)$ in a $(2|2)$--dimensional
complex superspace.
