Patent ID: 8669117

Claim:
A method for incorporating a single labeled oxygen atom into peptides, the method comprising: (1) forming a peptidase solution comprising: (a) 18 O enriched water containing at least 90% H 2 18 O; (b) at least a first peptidase selected from the group consisting of peptidyl-Lys metalloendopeptidases (EC 3.4.24.20), peptidyl-Asp metalloendopeptidases (EC 3.4.24.33), and combinations thereof; and (c) a buffer for raising a pH of the peptidase solution to a level at which the first peptidase incorporates only a single 18 O atom from the 18 O enriched water into at least 90% of the peptides; and (2) incubating the peptides with the peptidase solution for a period of time sufficient to incorporate only a single 18 O atom into at least 90% of the peptides.