if you have one of those old house cats lying around like i do (mine is like 19 years old and sleeps all day) throw that bitch in the skillet and make you some grade A protein with a high biological value.
I wouldn't worry about. The only concern should be whether you're getting complete proteins or not, because the resulting product - peptides and aminoacids - are identical, it's whether the given protein source has them or not that one should worry about.
Red meat and fish over any powder any day for me. Solid > liquid