Rat oocyte tissue plasminogen activator is a catalytically efficient enzyme in the absence of fibrin. Endogenous potentiation of enzyme activity.

Rat oocytes synthesize tissue plasminogen activator (tPA) in response to stimuli which initiate meiotic maturation. Purified tPA exhibits optimal activity only in the presence of fibrin or fibrin substitutes. Because oocytes are not exposed to fibrin in situ, we investigated the possible stimulation of rat oocyte tPA activity by other endogenous factor(s). Oocytes were obtained from immature female rats which were induced to ovulate with gonadotropins. tPA activity was measured by the plasminogen-dependent cleavage of a chromogenic substrate. Measurements of kinetic parameters with Glu- or Lys-plasminogen revealed a Km for the rat oocyte enzyme of 1.3-2.1 microM compared with 23-24 microM for purified human tPA. Inclusion of the soluble fibrin substitute polylysine lowered the Km of human tPA by 30-fold (0.8 microM) but had no effect on the oocyte tPA Km. Polylysine had no significant effect on the Vmax values. The rate of plasminogen activation catalyzed by oocyte tPA was increased only 4.3-fold by fibrin while fibrin stimulated purified human tPA activity by 15.2-fold. After fractionation of oocyte extract by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, polylysine enhanced oocyte tPA activity as seen by casein zymography. tPA activity in the conditioned medium of a rat insulinoma cell line was also not stimulated with polylysine prior to fractionation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. These data suggest that extravascular cells which elaborate tPA may produce stimulatory factor(s) which allow for full tPA activity at physiological concentrations of plasminogen in the absence of fibrin.