I am trying to isolate neuromelanin from samples of human brain by digesting out the rest of the brain using Proteinase K. Previously, I was using a 2h, 50C incubation with 10mg of fresh human brain (homogenised with pestle) and that successfully digested out the proteins in the brain leaving behind the neuromelanin which could be collected with a pellet.

However, this method does not seem to work for formalin fixed tissue as the brain does not seem to get digested. Is there anything I can do here? I was considering adding Tris base + SDS to the tissue first and then incubating with Proteinase K. Would this work? Are there any alternate methods or timings I should consider?