TRYPANOSOMATID CYSTEINE PROTEASE ACTIVITY MAY BE ENHANCED BY A KININOGEN-LIKE MOIETY FROM HOST SERUM

African trypanosomes contain cysteine proteases (trypanopains) the act ivity of which can be measured by in vitro digestion of fibrinogen, af ter electrophoresis in fibrinogen-containing SDS/ polyacrylamide gels. When assessed by this procedure, trypanopain from Trypanosoma brucei (trypanopain-Tb) is estimated to have a molecular mass of 28 kDa. Howe ver, two additional bands of trypanopain activity (87 kDa and 105 kDa) are observed if serum is added to the trypanopain before electrophore sis. Formation of the 87 and 105 kDa bands is frequently accompanied b y a reduction in the intensity of the 28 kDa activity which suggests t hat the extra bands are complexes of the 28 kDa trypanopain-Tb and a m olecule from rat serum called rat trypanopain moledulator (rTM). The r TM-induced activation of cysteine proteases is not restricted to T. br ucei as it is also observed with proteases from other protozoan parasi tes such as bloodstream forms of Trypanosoma congolense and the mammal ian-infective in vitro-derived promastigote forms of Leishmania donova ni and Leishmania major. The physical properties of rTM resemble those of the kininogen family of cysteine protease inhibitors. rTM is an ac idic (pI 4.7) heat-stable 68 kDa glycoprotein with 15 kDa protease-sus ceptible domains. This resemblance between rTM and kininogens was conf irmed by the positive, albeit weak, immunoreactivity between anti-(hum an low-molecular-mass kininogen) antibody and rTM as well as anti-rTM antibody and human low-molecular-mass kininogen. Furthermore, commerci al preparations of human-low-molecular-mass kininogen and chicken egg white cystatin mimicked rTM by forming extra bands of proteolytic acti vity in the presence of trypanopain-Tb. In some instances, low-molecul ar-mass kininogen was also observed to increase the rate of hydrolysis of 7-(benzyloxycarbonyl-phenylalanyl-arginyl- amido)-4-methylcoumarin by live T. brucei. Although this effect was rather erratic, in no ins tance was significant inhibition observed when this putative cysteine protease inhibitor was used under these conditions. The activation of parasite cysteine proteases by commonly accepted cysteine protease inh ibitors is unexpected and may have important pathological repercussion s.