Studies on the apoproteins of the major lipoprotein of the yolk of hen's eggs. V. Protein sulfhydryl groups: accessibility in the lipoprotein and the effect of temperature.
Sulfhydryl groups in the apoproteins of the major lipoprotein (density 0.95 g/ml) of hen's egg yolk have been studied by colorimetry, analysis for carboxymethylcysteine, and labelling with [14C]iodo-compounds and their distribution amongst the individual apoproteins (i.e. apovitellenins I-VI) has been determined. In the intact lipoprotein, reaction of sulfhydryl groups with aqueous reagents was slower than in the apoprotein mixture and there were large differences amongst different reagents in the extent of reaction. If the lipoprotein was heated for a short time in 1 mM EDTA above about 70 degrees C and then cooled, more sulfhydryl groups were accessible although the total number did not change. This increase in accessibility was accompanied by the disruption of the lipoprotein structure and an increase in the viscosity of its solutions. Furthermore, the isolated apoprotein mixture was less soluble in urea solutions and formed a gel, probably as a result of cross-linking by inter-protein disulfides. There was also a small change in the distribution of sulfhydryl groups so it is likely that sulfhydryl-disulfide interchanges occurred on heating. It is suggested that such interchanges help determine the physical properties of heated yolk and that in the intact lipoprotein sulfhydryl groups are concealed to prevent sulfhydryl-disulfide interchanges in the egg.
D000818:Animals; D001059:Apoproteins / Q000032:analysis*; D002645:Chickens; D004220:Disulfides / Q000032:analysis; D004228:Dithionitrobenzoic Acid; D004527:Egg Proteins / Q000032:analysis*; D004530:Egg Yolk / Q000032:analysis*; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D008074:Lipoproteins / Q000032:analysis; D008301:Maleimides / Q000494:pharmacology; D013438:Sulfhydryl Compounds / Q000032:analysis*; D013696:Temperature; D014783:Viscosity
D001059:Apoproteins; D004220:Disulfides; D004527:Egg Proteins; D008074:Lipoproteins; D008301:Maleimides; D013438:Sulfhydryl Compounds; C016399:N-(4-dimethylamino-3,5-dinitrophenyl)maleimide; D004228:Dithionitrobenzoic Acid