Assay of inhibition of LAP by antibodies.An in vitro inhibitory assay was developed to investigate the effect of antibodies induced in the vaccinated sheep on the LAP activity. Protein A affinity column-purified IgG preparations from infected and control animals were assayed as follows. A 100-μl volume of LAP (60 μg) was preincubated with 100 μl of purified IgG (50 μg) at 37°C for 1 h. A 100-μl volume of this reaction mixture was then added, in duplicate, to wells of polystyrene microplates preloaded with 100 μl of 0.1 M glycine buffer, pH 8.3. The plate was then incubated for 10 min at 37°C prior to the addition of 50 μl of 4 mM L-leucine p -nitroanilide (pNA) to each well. LAP incubated without purified IgG served as a control. The OD405 was determined at time zero and following a 2-h incubation at 37°C, and the amount (in nanomoles) of pNA released per minute per milligram of protein was estimated as previously described (19). Protein concentrations were determined by using a Micro-BCA protein assay kit (Pierce, Rockford, Ill.) (22).