The effect of iodoacetamide (IA), a sulphydryl blocking agent, on fertilisation in the mouse was examined by transferring zona-free oocytes into IA (500 μM)-containing medium at various times after insemination. When inseminated oocytes were transferred into IA medium 10 min after insemination, the oocyte chromosomes remained aggregated in one or two masses and the sperm nucleus failed to decondense. When oocytes were transferred during the second meiosis, oocytes meiosis was more or less arrested. The sperm nucleus decondensed but remained blocked at an early stage of decondensation. These observations suggest that thiol groups in the oocyte's cytoplasm and perhaps microtubules of the meiotic spindle play crucial roles in the completion of meiosis and the transformation of sperm nucleus into pronucleus.

In amphibian eggs the formation of a capsular chamber is one of the most striking events occurring either upon oviposition or after fertilisation. In the egg of the anuran Discoglossus pictus a capsular chamber forms following fertilisation or activation; the egg with its vitelline envelope rotates in this chamber according to gravity. Previous work showed that the chamber is the product of plug dissolution. The plug is a lens-shaped jelly coat, typical of Discoglossus, covering only part of the animal hemisphere. Its dissolution is caused by material released from the egg about 15 min after fertilisation through exocytosis of at least two types of vacuoles. Liquefaction of the plug correlates with the reduction of disulphide bonds present in the jelly matrix. In this study we investigated the nature of the substances released from the egg and some changes occurring in the plug during liquefaction. SDS-PAGE showed that the proteic profile of the plug changes dramatically after fertilisation, confirming proteic cleavage in the plug matrix during its dissolution. Through in vitro tests and electrophoretic analysis of the Ringer solution in which the egg exudate was collected, an increase in the activity of the solution was determined in the presence of hydrogen peroxide, and peroxidase activity was depicted in the egg exudate. The presence of free thiol groups and cysteic acid residues (or cysteine sulphinic acid) in the plugs of activated eggs was established, suggesting that during plug dissolution some disulphide bonds are oxidatively opened. This suggests that enzyme(s) with peroxidase activity are released following fertilisation. We surmise that such enzymes are contained in the intraovular vacuoles the exocytosis of which triggers the onset of plug liquefaction. The possible release of hydrogen peroxide from the egg is discussed.

Heat-induced gelation (80 °C, 30 min or 85 °C, 60 min) of whey protein concentrate (WPC) solutions was studied using transmission electron microscopy (TEM), dynamic rheology and polyacrylamide gel electrophoresis (PAGE). The WPC solutions (150 g/kg, pH 6·9) were prepared by dispersing WPC powder in water (control), 10 g/kg sodium dodecyl sulphate (SDS) solution or 10 mM-dithiothreitol (DTT) solution. The WPC gels containing SDS were more translucent than the control gels, which were slightly more translucent than the gels containing DTT. TEM analyses showed that the SDS-gels had finer aggregate structure (≅10 nm) than the control gels (≅100 nm), whereas the DTT-gels had a more particulate structure (≅200 to 300 nm). Dynamic rheology measurements showed that the control WPC gels had storage modulus (G′) values (≅13500 Pa) that were ≅25 times higher than those of the SDS-gels (≅550 Pa) and less than half those of the DTT-gels after cooling. Compression tests showed that the DTT-gels were more rigid and more brittle than the control gels, whereas the SDS-gels were softer and more rubbery than either the control gels or the DTT-gels. PAGE analyses of WPC gel samples revealed that the control WPC solutions heated at 85 °C for 10 min contained both disulphide bonds and non-covalent linkages. In both the SDS-solutions and the DTT-solutions, the denatured whey protein molecules were in the form of monomers or small aggregates. It is likely that, on more extended heating, more disulphide linkages were formed in the SDS-gels whereas more hydrophobic aggregates were formed in the DTT-gels. These results demonstrate that the properties of heat-induced WPC gels are strongly influenced by non-covalent bonding. Intermolecular disulphide bonds appeared to give the rubbery nature of heat-induced WPC gels whereas non-covalent bonds their rigidity and brittle texture.