Recombinant L-aminoacylase (PhoACY) from a hyperthermophilic archeon, Pyrococcus horikoshii, is a zinc-containing metalloenzyme. When the zinc was substituted by Mn2+ or Ni2+, its specific activity was significantly increased with acetyl-L-methionine as a substrate. The thermostability of PhoACY was improved when it was incubated with 1 mM Zn2+, Mn2+ or Ni2+. The enzyme with external Zn2+ addition had no significant loss of the activity when held at 90 degrees C for up to 12 h and moreover had more than a 10-fold longer half-life even at 100 degrees C, compared to the enzyme without Zn2+ addition. A thermostable structure of the enzyme associated with zinc binding is described based on differential scanning calorimetry.