HKH20 inhibits S pyogenes-induced contact activation. (A) Human plasma was incubated with S pyogenes bacteria in the presence of 50 μM HKH20, GCP28, or buffer alone (control) for 30 minutes. Plasma was removed by centrifugation, and bacteria were washed and resuspended in substrate buffer. After 30 minutes of incubation, plasma kallikrein activity was measured in a substrate assay. Data are presented as percentage activity compared with the control; values are mean plus or minus SD (n = 3). *P < .05 by t test. (B) Human plasma was incubated with S pyogenes bacteria in the presence or absence of HKH20 or GCP28 for 15 minutes. Bacteria were washed, resuspended in buffer, incubated for 15 minutes, and spun down. Supernatants and plasma (nontreated or kaolin-treated) were analyzed by SDS-PAGE and Western blotting with antibodies against HK and LK: lane 1, normal plasma; lane 2, kaolin-treated plasma; lane 3, plasma proteins absorbed and released by S pyogenes; lane 4, plasma proteins absorbed and released by S pyogenes in the presence of 100 μM HKH20; lane 5, plasma proteins absorbed and released by S pyogenes in the presence of 100 μM GCP28. Vertical lines have been inserted to indicate a repositioned gel lane.