Angiotensin-converting enzyme and enkephalinase in human breast cyst fluid

Palpable breast cysts with an apocrine epithelial lining (type 1) are reported to be associated with a higher risk of developing breast cancer. The composition of breast cyst fluid (BCF) might include those factors involved in this increased risk. In this study peptidase activities that were active against the substrate [125I]metenkephalin-Arg-Phe were detected in BCF. The products were identified by reversed phase high-performance liquid chromatography (HPLC) as [125I]Tyr-Gly-Gly and [125I]Met-enkephalin. This proteolysis was not inhibited by PCMB, pepstatin A, leupeptin or aprotinin but was by EDTA, showing that the activity was due to metalloproteases. The production of [125I]Try-Gly-Gly was inhibited by phosphoramidon and thiorphan, whereas that of [125I]met-enkephalin was inhibited by captopril and Bothrops jararaca peptide, indicating that these activities are enkephalinase and angiotensin-converting enzyme (ACE) respectively. A fluorometric assay for ACE demonstrated that ACE levels are significantly higher in type 2 BCF than in type 1 BCF (30.8 vs 6.1 nmol hr-1 10 microliters-1, P < 0.001). As the increased risk of cancer is linked to type 1 cysts it is possible that higher levels of peptidase in type 2 BCF reflect a protective environment in the breast in which mitogenic peptide growth factors are neutralised by proteolysis.