Death Defying by Nina Croft

Captain Tannis of the starship El Cazador is part snake, part human, and--according to some--all bitch. She is one job away from her lifelong goal--to achieve the immortality she craves. The assignment: protecting the most powerful man in the Universe.

After five hundred years, Callum Meridian, founding member of the Collective, is bored out of his mind. Until he realizes he's physically changing--into what, he isn't sure. Callum is determined to discover the truth, but his own people will stop at nothing to prevent it from coming out.
With a coup brewing and the crew of El Cazador close to tossing him off the ship, Callum turns to Tannis. Sparks fly as they work together to make it out alive. But can Callum really trust the one woman hell bent on using him? Defying death has never been more dangerous, more sexy, or more fun.