Isolated DNA encoding protein having improved stability

An isolated DNA encoding a protein which has NADH oxidase activity or NADPH oxidase activity or both and has improved stability compared to the protein having the amino acid sequence of SEQ ID NO:

• 1, and wherein said protein has an amino acid sequence that has at least 95% sequence identity to the amino acid sequence of SEQ ID NO:
• 1 and further contains at least one amino acid substitution selected from (a) to (g):
• (a) a substitution of an amino acid residue at a position corresponding to position 42 of SEQ ID NO:
• 1 with an amino acid having a side-chain surface area of 100 to 200 Å2;
• (b) a substitution of an amino acid residue at a position corresponding to position 46 of SEQ ID NO:1 with a neutral amino acid having a side-chain surface area of 100 to 150 Å2 or an acidic amino acid having a side-chain surface area of 100 to 150 Å2;
• (c) a substitution of an amino acid residue at a position corresponding to position 96 of SEQ ID NO:1 with a basic amino acid;
• (d) a substitution of an amino acid residue at a position corresponding to position 172 of SEQ ID NO:
• 1 with an amino acid having a smaller side-chain surface area than Tyr;
• (e) a substitution of an amino acid residue at a position corresponding to position 196 of SEQ ID NO:1 with a basic amino acid;
• (f) a substitution of an amino acid residue at a position corresponding to position 312 of SEQ ID NO:
• 1 with an amino acid having a larger side-chain surface area than Ala;
• and
• (g) a substitution of an amino acid residue at a position corresponding to position 371 of SEQ ID NO:
• 1 with an aliphatic amino acid, an acidic amino acid, or an amino acid having a hydroxyl group-bearing side chain.