HLMs 16 and 20 were preincubated with indicated amounts of MAB-2E1 for 15 min before initiation of the 30-min incubation by addition of 12 mM BUP, buffer, and the NADPH-generating system. Control mixtures consisting of HLMs and 25 mM Tris buffer in the absence of MAB-2E1 were also performed. Rates of HBUP formation in the presence of antibody were determined in duplicate and expressed as the percentage of control activity.