The vitelline envelope-to-fertilization envelope transformation in the toad Bufo arenarum

An investigation of some changes associated with the transformation of the vitelline envelope into the fertilization envelope in the egg of the toad Bufo arenarum is reported here. In most of the experiments described, the parameter used to demonstrate these changes was the stability of structural integrity of isolated envelopes when submitted to different agents and conditions. The envelopes used for this purpose exhibited a high degree of purity and remained apparently unaltered by the isolation procedure. As a quantitative method to ascertain their solubility rate, the release of uv-absorbing materials into solution was determined. Compared to the vitelline envelope, the fertilization envelope has proven to be less soluble in water, more stable in the presence of the chaotropic ion thiocyanate, and less susceptible to digestion in the presence of sperm lysin, trypsin, and pronase. In Bufo arenarum, as in other species, the vitelline envelope appears to be composed of glycoproteins. In contrast to previous results, however, disulfide bonds do not seem to be involved in their structural integrity. Thus, experiments carried out with isolated envelopes as well as with envelopes in situ have demonstrated a lack of effect of disulfide bond breaking agents on envelope stability. Evidence is presented suggesting that the solubility of envelopes in mercaptan solutions, as reported by other laboratories, is likely to be the expression of artifactual results.