Histochemical method for characterization of enzyme crystals: application to crystals of Torpedo californica acetylcholinesterase

Histochemical methods are employed to detect and localize a wide range of enzymes. Even though protein crystallographers do not commonly use this technique, the extensively used colorimetric reaction of Karnovsky was successfully adapted for easy and quick identification of acetylcholinesterase crystals. The method relies on the reduction of ferricyanide to ferrocyanide by thiocholine, released from acetylthiocholine by enzymatic hydrolysis, followed by formation of a cupric ferrocyanide precipitate, and allows rapid differentiation between salt and enzyme crystals and between native and inhibited crystals of the enzyme.