Superpotency and superaffinity phenomena in the stimulation of steroidogenesis in adrenocortical cells by adrenocorticotropin-tobacco mosaic virus conjugates.

Adrenocorticotropin-(1-24)-tetracosapeptide was covalently attached to tobacco mosaic virus in two different manners: (i) through a handle near the C-terminus on tyrosine-(23) and (ii) through a handle at the N-terminus on serine-(1). Compounds of type (i) with their N-terminal message sequence freely exposed on the virion surface were considerably more potent for stimulating steroidogenesis in isolated adrenocortical cells than those of type (ii) with a more congested message. Conjugates with 50 or less hormone molecules per virion were less potent per peptide unit than the "free" handle-substituted hormones, whereas conjugates with 150 ACTH units exhibited superpotency effects. Superpotency disappeared when the substituted virions were disaggregated into (substituted) capsomers, suggesting influences of hormone clustering and virion geometry on biological activity. Superpotent stimulation was irreversible under conditions that immediately inhibited steroidogenesis by ACTH (dilution, addition of a peptide antagonist). Thus, superpotency might be caused by superaffinity arising from a slow rate of dissociation of the conjugates from the target cell receptors. The reason for the slow dissociation rate is still unclear: possible explanations include cooperative affinity, rapid internalization of the conjugate-receptor complexes, or decreased rates of peptide degradation at the receptor site.