Hormonal properties of avidin-biotinylinsulin and avidin-biotinylcorticotropin complexes

In connection with the development of affinity columns (based on the avidin-biotin interaction) for retrieval of peptide and protein hormone receptors, the hormonal properties of a number of avidin-biotinylinsulin and avidin-bioinylcorticotropin complexes were examined. Of particular interest was an evaluation of streptavidin as a ligand for the attachment of biotinylated hormones to solid supports and its possible advantage over SpHPP-avidin (S = succinoylated; pHPP = 3-(p-hydroxyphenyl)propionyl). As concerns binding kinetics using rat liver plasma membranes, streptavidin was found superior to avidin since it does not display apparently nonsaturable binding. Scatchard analyses of the binding of 125I-streptavidin, 125I-S-streptavidin and 125I-SpHPP-avidin to rat liver plasma membranes gave KD value of 6.7, 13.2, and 10.6 nM respectively. The binding was saturable and the unlabeled proteins competed with their labeled counterparts for the membrane binding sites. Biotinylinsulin, attached to either streptavidin or SpHPP-avidin was able to compete for 125I-insulin-binding sites on rat liver plasma membranes though somewhat larger concentrations of the complexes than of insulin were required to achieve comparable inhibition. The ID50 values for insulin and the biotinylinsulin complexes were 5 and 80 nM respectively. Biotinylcorticotropin was found to be a more effective activator of particulate rat adrenal adenylate cyclase when complexed with unmodified avidin than with streptavidin, S-streptavidin or SpHPP-avidin.