According to Steven Rudich, a student at Carnegie-Mellon suggested using A=K, the halting problem. Does this work? This led to an interesting email discussion between Rudich, Richard Beigel, Lane Hemaspaandra and few more of us getting cc'd on the messages.
We need to use a reasonably dense encoding of K, such as the set of Java programs with no input that halt. Then in fact PK does equal NPK, but the proof is not as obvious as one would imagine. Think about it.
And if anyone knows the original reference to PK=NPK, let us know.