Isolation of quiescin-sulfhydryl oxidase from milk
Methods for isolating a highly purified QSOX enzyme from milk or whey are described. The enzyme may be used to generate intermolecular and intramolecular disulfide bridges and to facilitate oxidative protein folding.
1. An isolated and purified QSOX polypeptide having at least 95% identity to the amino acid sequence of SEQ ID NO: 1, wherein the isolated and purified polypeptide is capable of inserting a disulfide bond into a peptide or polypeptide.
2. The isolated and purified QSOX polypeptide of claim 1 , wherein the polypeptide consists of a polypeptide having the amino acid sequence of SEQ ID NO:1.
3. A method for isolating and purifying the QSOX polypeptide of claim 1 from milk comprising the steps of
a) preparing a solution of whey from the milk,
b) acidifying the solution of whey,
c) applying the acidified solution of whey to a cation exchanger,
d) eluting and identifying fractions containing the QSOX polypeptide from the cation exchanger, and
e) further purifying the QSOX polypeptide in the eluted fractions.
4. The method of claim 3 , wherein step e) comprises subjecting the eluted QSOX polypeptide to hydrophobic interaction chromatography.
5. The method of claim 4 , further comprising the step of subjecting the QSOX polypeptide to a second cation exchanger, wherein the second cation exchanger has higher column efficiencies for chromatographic polishing than the cation exchanger of step c).
6. The method of claim 3 , wherein the isolated QSOX polypeptide is a flavin-dependent quiescin sulfhydryl oxidase.
7. The method of claim 5 , wherein the isolated QSOX polypeptide is at least 90% pure.
8. The method of claim 3 , wherein the milk is bovine milk.
9. The method of claim 8 , wherein the isolated QSOX polypeptide is isolated in quantities of at least 0.27 mg/liter of milk.
10. The method of claim 8 , wherein the isolated QSOX polypeptide is isolated in quantities of at least 3.3 mg/100 g whey protein.
11. The method of claim 3 wherein the isolated QSOX polypeptide comprises the amino acid sequence of SEQ ID NO:1.
12. A method for isolating and partially purifying the QSOX polypeptide of claim 1 from a solution of whey comprising the steps of
a) acidifying the solution of whey,
b) applying the acidified solution of whey to a cation exchanger,
c) eluting and identifying fractions containing quiescin sulfhydryl oxidase from the cation exchanger, and
d) further purifying quiescin sulfhydryl oxidase in the eluted fractions.
13. The method of claim 12 , wherein step d) comprises subjecting the isolated QSOX polypeptide to hydrophobic interaction chromatography.
14. The method of claim 13 , further comprising the step of subjecting the isolated QSOX polypeptide to a second cation exchanger, wherein the second cation exchanger has higher column efficiencies for chromatographic polishing than the cation exchanger of step b).
15. The method of claim 12 , wherein the isolated QSOX polypeptide is a flavin-dependent quiescin sulfhydryl oxidase.
16. The method of claim 14 , wherein the isolated quiescin sulfhydryl oxidase is at least 90% pure.
17. The method of claim 12 , wherein the whey is bovine whey.
18. The method of claim 17 , wherein the isolated QSOX polypeptide is isolated in quantities of at least 3.3 mg/100 g protein from whey.
19. The method of claim 12 , wherein the isolated QSOX polypeptide comprises the amino acid sequence of SEQ ID NO:1.