matrix in \frac{}{}

[Bodo Steiner]

Hi, I'm trying to get this running, but it won't ... how can I have a 
matrix in the following formula:

\begin{equation}
\alpha^{\star}=\frac{\mbox{\boldmath$z$}^{T}\mbox{\boldmath$\Lambda$}
\mbox{\boldmath$\Sigma$}}{\mbox{\boldmath$z$}^{T}\mbox{\boldmath$\Lambda$}
\begin{bmatrix}-\mbox{\boldmath$K$}_{1}^{-1}&0\\
0&\mbox{\boldmath$K$}_{2}^{-1}
  \end{bmatrix}\mbox{\boldmath$\Lambda$}^{T}\mbox{\boldmath$z$}
  +r\mbox{\boldmath$z$}^{T}\mbox{\boldmath$\Lambda$}
  \mbox{\boldmath$\Sigma$}\mbox{\boldmath$\Lambda$}^{T}{\mbox{\boldmath$z$}}
\end{equation}

thanks!
-bodo.