How can you stabilize enzymes in pellets made from microcrystalline cellulose?
As you can probably guess from my comment, there are a lot of factors to take into account when thinking about protein stability. Here's what I would recommend you do.

First, make sure your extract is in a neutral buffer, like PBS, HEPES, or something similar. This is a fairly standard procedure when working with proteins. Next, since this is a kidney extract, there are likely to be gobs of proteases around, so I would strongly recommend adding broad-spectrum protease inhibitors, and keeping the extract as cold as possible to inhibit any enzyme activity. I would also add BSA at 10 mg/ml, both to stabilize the proteins and keep them from precipitating while in solution, and to lessen the chances that any proteases present will attack your enzyme of interest.

Once you have your solution stabilized, I would aliquot it and lyophilize it. Proteases are most active in solution, so once dried you'll reduce their activity even more. You can now add the inert ingredients (microcrystalline cellulose, etc.), mix thoroughly, press your tablets, and add the enteric coating. Finally, once everything is prepared, I would store the final product at as cold a temperature as is convenient. Again, this will inhibit any enzyme activity and prevent degradation.

I don't know if you have a functional assay to determine activity of the enzyme of interest, but if you don't I'd highly recommend developing one if at all possible. This way, you can test the activity of the fresh preparation of extract and compare it to various stages of processing into tablets, especially the finished goods, to ensure that you haven't lost much activity. I would also test random tablets throughout the year to make sure they haven't lost activity in storage, otherwise you won't be able to compare your experimental results.