A new method to evaluate the enzyme-suppressing activity of a leucine aminopeptidase 3 inhibitor

The expression of leucine aminopeptidase 3 (LAP3) is associated with the prognosis for and malignant transformation of many types of tumors. Therefore, a LAP3 inhibitor may represent a new strategy for cancer therapy. Evaluating the suppression of enzyme activity by an LAP3 inhibitor is essential. Right now, leucine aminopeptidases (LAPs) purified from the porcine kidneys are the only enzymes that can be used to evaluate the suppression of enzyme activity by an LAP3 inhibitor. This approach cannot accurately reflect the suppression of human LAP3 by an inhibitor. The current study developed a new method with which to evaluate the suppression of enzyme activity by an LAP3 inhibitor. Total protein from K562 cells seldom catalyzed the LAP3 substrate. A lentivirus was used to induce K562 cells to overexpress LAP3 (K562-LAP3). After puromycin screening, flow cytometry data indicated that 98.8% of cells expressed green fluorescent protein. The expression of LAP3 in K562-LAP3 cells was also assessed using Western blotting. K562-LAP3 cells were lysed with ultrasonication. Total protein was used as an enzyme source and L-leucine p-nitroaniline hydrochloride was used as a substrate to measure enzyme activity. Total protein from K562-LAP3 cells catalyzed the substrate more than that from K562 cells did. The LAP3 inhibitor ubenimex was used as a positive control to evaluate the suppression of LAP3 enzyme activity. Results indicated that ubenimex significantly inhibited the enzyme activity of LAP3. This approach provides a convenient and accurate way to evaluate the suppression of enzyme activity by an LAP3 inhibitor.