Purification of human kallikrein 6 from biological fluids and identification of its complex with alpha(1)-antichymotrypsin.

BACKGROUND Human kallikrein 6 (hK6) is significantly increased in serum in many patients with ovarian cancer and may have a role in amyloid precursor processing and Alzheimer disease. The forms of hK6 in biological fluids are poorly characterized. METHODS hK6 protein was immunoaffinity-purified and positively identified by Western blotting, N-terminal sequencing, and mass spectrometry. hK6 in cerebrospinal fluid (CSF), milk, ascites, and serum was size-fractionated by chromatography and then measured by a highly sensitive and specific immunoassay. Hybrid assays were performed to detect the possible interactions between hK6 and proteinase inhibitors in CSF, milk, ascites fluid, and serum. RESULTS N-Terminal sequencing identified hK6 in the proform in both CSF and milk. hK6 exists in two forms in milk and ascites fluid: a free form with a molecular mass of approximately 25 kDa and a higher molecular mass form. Hybrid sandwich assays (capture antibody for hK6 and detection antibody for inhibitors), utilizing a panel of known serine protease inhibitors, indicated that alpha(1)-antichymotrypsin forms a complex with hK6 in milk and ascites fluid. Only the free form of hK6 was detected in CSF and serum. CONCLUSIONS hK6 exists mainly as a proenzyme in milk and CSF. A fraction of this enzyme is partially complexed with alpha(1)-antichymotrypsin in milk and ascites fluid of ovarian cancer patients.