Induction of maturation in Xenopus laevis oocytes by a steroid linked to a polymer.

A progesterone analog has been covalently linked via an amide bond to polyethylene oxide (molecular weight, 20,000). This macromolecular steroid molecule displays the biological activity of progesterone in inducing meiotic maturation when incubated with Xenopus laevis oocytes (stage VI) in vitro. Its efficiency (half-maximum effective concentration, 30 muM) is approximately 10 times lower than that of its low molecular weight homolog (3 muM). Control experiments with polyethylene oxide and an estradiol derivative (up to 1 mM) assessed the specificity of the progesterone macromolecular analog. Uptake experiments using radioactive derivatives revealed a small (if not negligible) intake of the macromolecular progesterone analog by the oocytes compared to that of free steroids, and no parallelism was found between radioactivity incorporation and effect. The possibility of cleavage of the macromolecular derivative during the incubation was ruled out. Furthermore, injection of the polymer-linked progesterone into the oocytes did not induce maturation. These observations suggest that the macromolecular progesterone analog itself is responsible for the biological effect and that the presence of this compound inside the cell is neither necessary nor sufficient for triggering reinitiation of meiosis. These conclusions are in agreement with the proposal that interaction with the plasma membrane of the oocyte is necessary for progesterone action in this particular system, in contrast to the case of somatic cells which have intracellular steroid receptors.