Bovine lysyl oxidase (BLO) contains two different cofactors, copper (Kagan, H. M. 1986, Biology of Extracellular Matrix , 1, 321-398) and lysine tyrosyl quinone (LTQ) (Wang, S. X., et al, 1996, Science, 273, 1078-1084). By a combination of UV-Vis spectroscopy, metal content analysis, and activity measurements, we find that copper depleted BLO reacts in an irreversible manner with phenylhydrazine, an amine substrate analog, and catalyzes multiple turnovers of the substrate benzylamine. After removal of the majority of enzyme-bound copper, apo-BLO exhibits a decrease in the LTQ content, as evidenced by the drop of the 510-520 nm absorbance, suggesting that the copper may play a structural role in stabilizing the LTQ. The remaining intact LTQ in the apo-BLO reacted with phenylhydrazine, both in the presence and absence of the chelator, 10 mM 2, 2'-dipyridyl. When benzylamine was used as the substrate, the apo-BLO turned over at a rate of 50-60% of the native BLO (after correction for the residual copper and the change of LTQ content). Copper contamination from the assay buffer was ruled out by comparison of enzyme activity using different apo-BLO concentrations. These studies demonstrate that the mature form of lysyl oxidase retains many of its functions in the absence of copper.