... 4 Cytotoxicity on the HL-60 cell line. Conclusions. ... As a result, the moiety required for the antioxidant activity of UA was 2-CO and 7-NH or 9-NH because 5 and 6 had greater antioxidant activity than UA. Introduction of 3-NH, 8-CO, and 9-NH moieties to 5 increased the activity. ...