Vitellogenin synthesis and characterisation of the liver estrogen receptor in the neotenous salamander Ambystoma mexicanum

Abstract

The neotenous salamander Ambystoma mexicanum reaches sexual maturity without completing metamorphosis. Females must, therefore, synthesize vitellogenin, the precursor of the egg-yolk proteins. We show that livers of female axolotls synthesize and secrete a phosphoprotein which migrates with Xenopus vitellogenin on SDS-gels and is precipitated by antibody prepared against Xenopus vitellogenin. The livers of male axolotls do not normally synthesize this protein but can be induced to do so by treatment in vivo with estradiol. A receptor with a high affinity for estradiol ( K d = 0.3 × 10 −9 M) was found in the nuclei prepared from livers of male and female axolotls. It sediments at 3.7 S at 0°C in sucrose gradients containing 0.5 M KCl. Each nucleus contains about 1300 binding sites for estradiol, 13 times the number found in normal male Xenopus nuclei, but as axolotl nuclei are about 12 times larger, the concentrations of binding sites are similar. In contrast to Xenopus, there is no detectable increase in the number of nuclear binding sites following estrogen treatment. We conclude that the controls affecting both the appearance of vitellogenin inducibility and the induction of vitellogenin synthesis differ between the two species A. mexicanum and Xenopus laevis.