Given that the partial rate
$$
\Gamma\left(\mathrm{K}^{+} \rightarrow \pi^{0} \mathrm{e}^{+} \nu\right)=4 \times 10^{6} \mathrm{sec}^{-1},
$$
calculate the rate for $\mathrm{D}^{0} \rightarrow \mathrm{K}^{-} \mathrm{e}^{+} \nu$. Hence, estimate the lifetime of the $\mathrm{D}^{0}$ meson.