Selective Inhibition of Cleavage in Different Regions of the Frog Egg by Sulphydryl Inhibitors

When frogs are exposed during early cleavage to low concentrations of the sulphydryl inhibitor, p-chloromercuribenzoic acid, cleavage is inhibited at the animal pole, while the marginal zone and yolk cleave normally. Eggs exposed before first cleavage cleave only at the animal pole, while blastulae and gastrulae are inhibited uniformly in all regions by the sulphydryl reagent. The selective inhibition of animal-pole cells by p-chloromercuribenzoic acid is not readily explained on the basis of regional permeability differences or variations in sensitivity of the surface coat to inhibitors, since exposure of eggs to iodoacetamide results in normal cleavage at the animal pole, together with inhibition of cleavage at the marginal zone and in the yolk. The inhibition produced by p-chloromercuribenzoic acid is mediated by sulphydryl groups, since the effects of the inhibitor are reversible by thioglycollate and cysteine. When various protein fractions are extracted from eggs or hatched embryos and tested for sulphydryl groups by means of the nitroprusside reaction, the soluble fractions are found to contain freely reactive sulphydryl groups, while the yolk proteins are entirely lacking in reactive protein-bound sulphydryl groups. When the yolk proteins are fractionated into phosphoprotein (S2) and non-phosphoprotein (Si), the latter alone is found to contain masked SH groups that become fully reactive only after denaturation plus reduction of disulphide groups. It is suggested that the protein-bound SH groups at the marginal zone are protected against p-chloromercuribenzoic acid inhibition by a high content of soluble thiols such as reduced glutathione, which are synthesized at the expense of sulphur-containing amino acids from the yolk protein Si, under the control of enzymes and nucleotides such as adenosine triphosphate present in the dorsal lip region and marginal zone.