Activation of human topoisomerase I by protein kinase CK2

The enzymatic studies were performed to reveal a mode of activation of human topoisomerase I by a direct interaction with protein kinase CK2. In the absence of ATP CK2 kinase activated DNA relaxation about twofold. CK2 subunit was identified as solely responsible for the stimulation of relaxing activity by CK2 kinase. CK2 activated the relaxation only at the excess of the substrate over topoisomerase I. At the equimolar ratio of the substrate DNA and topoisomerase I the activation was not observed. There was also no effect of CK2 on camptothecin-induced cleavage of DNA by htopo I. These results identify an accelerated movement of topoisomerase I between substrate molecules as a cause of the activation of DNA relaxation by CK2 kinase.     

The activation by staphylokinase of human plasminogen.

The activation of human plasminogen by a highly purified staphylokinase was investigated using casein or an active site titrant (p-nitrophenyl-p-guanidinobenzoate, NPGB) as a substrate. The reaction rate was time dependent, showing a pronounced lag period with either substrate. Saturation curve estimated from the caseinolytic assay was sigmoid, but changed to quasi-hyperbolic in the presence of pre-formed human plasmin. With NPGB, the extent of plasminogen conversion into esterolytic plasmin was directly proportional to staphylokinase concentration, and the saturation point was reached when the molar concentration of staphylokinase equaled that of plasminogen. It is concluded that staphylokinase acts stoichiometrically, forms an equimolar complex with plasminogen, and thus is not an enzyme but a modifier. Staphylokinase-activated plasminogen exhibits properties of a hysteretic enzyme.