Which would depend on your specific protein, and prions aren‘t that specific.

Sure globulins will be gone and other unstable protein, but since you didn‘t mention the protein.

Can just pasteurise in an autoclave as well.

No need to do 121C if you aren’t going for medical sterility. Most stuff does way earlier.

Well phosphate and protease resistant protein don‘t make up a very good growth mediums

So not much risk of colonies forming anyway.

Though got any further attempts you got stuff like propylenglykol or regular preservatives available. No use to go toxic.

Could even just use thiomersal if you still got some lying around.

But sorbic acid if acidic or parabenes if neutral to whatever if basic. That stuff works for creams that people touch daily just fine to prevent growth.

Other way round, find someone with an x ray in a lab and just use that.