FIGURE 3.

Identification of cleavage products of OPN purified from human urine. OPN was purified from urine, and the isolated protein batch consists of both OPN fragments as well as the full-length protein. The urinary OPN forms were digested with Asp-N in a buffer containing 50% H₂¹⁸O and analyzed by MS without prior separation of the resulting peptides. Two peptides corresponding to Asp¹⁴⁵–Arg¹⁵² and Asp¹⁴⁵–Lys¹⁵⁴ were identified as C-terminal peptides, whereas other OPN peptides were observed with the monoisotopic pattern characteristic of internal peptides.