Hello Bob 

It depend on the purpose of your study, if you want to do measuring respiratory chain enzymatic activities this will be fine, but for functional study you need to have mitochondria with intact membranes .

Thanks Mohammed, purpose is just to see in what fractions some proteins are enriched. Its possible for both frozen and fixed?

In my opinion, all membranes will be disrupted therefore, no intact organelles can be isolated and no membrane-linked enzymatic activities can be assessed.  Please provide a specific assay that you would like to perform so that we can provide guidance.

Cheers

Michael