Secreted NDPK-B exhibits transphosphorylation activity. Partially purified NDPK-B was incubated for 2 min with GTP (300µM) and ADP (0–60µM). An equal volume of luciferin-luciferase ATP detection buffer (Sigma, St. Louis, MO) was added and, following a 10 s delay, a single measurement of luminescence was recorded by luminometer. Relative luminescence units (RLU) were corrected from background and converted to ATP by comparison to a standard curve. Data are presented as mean ± SEM, n=3.