Avidin, a positively charged egg-white protein, aggregates extensively when mixed at ambient temperatures with anionic detergents, such as sodium dodecyl sulfate (SDS). The resultant aggregates fail to penetrate the stacking gel during polyacrylamide gel electrophoresis (PAGE). To prevent the formation of such aggregates, avidin was acetylated and the pI was thus reduced. Acetylated avidin was found to behave in a manner similar to that of streptavidin; under nondenaturing conditions (i.e., incubation of samples at room temperature), both proteins normally migrated mainly as tetramers with a tendency to form oligomers of the tetramer. When samples were boiled, both proteins migrated mainly as the monomer. The comparative stability properties of avidin and streptavidin were also examined using SDS-PAGE by heating samples and determining the extent of dissociation of tetramers to monomers as a function of temperature. A distinctive transition temperature could be defined for individual samples. Using this assay, it was determined that, in the absence of biotin, the quaternary structure of streptavidin is more stable than that of avidin. Biotin appears to stabilize structures of both avidin and streptavidin to a similar degree. Acetylation of avidin thus provides a simple means to analyze the quaternary structure of the molecule using SDS-PAGE.

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synopsisThe preparation of L-lysine peptides (Lys,, n = 2-14) from poly-L-lysine is described.Fractionation by ion-exchange column chromatography of poly-Llysine hydrolysates on apreparative scale resulted in 0.2-1.0g quantities of individual members of the poly-Llysine series. The peptides isolated proved to be analytically pure and the optical configuration was fully retained, as demonstrated by complete enzymic digestion. Peptides higher than n = 14 were also prepared. They cousist,ed of oligolysine groups of narrow and accurately determined size distribution. Potentiometric t.itratioris were used both to characterize the products and to demonstrate t.he charact~eristic dependence of the dissociation constants on size of the peptide. * A preliminary report has appealed.'

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