IMMUNOCYTOCHEMISTRY BY ELECTRON SPECTROSCOPIC IMAGING USING A HOMOGENEOUSLY BORONATED PEPTIDE

A linear all-L-oligopeptide containing five carboranyl amino acids (co rresponding to 50 boron atoms) was synthesized and specifically attach ed to the free thiol group of monovalent antibody fragments F(ab)'. Th e boronated immunoreagent was used for the direct post-embedding detec tion of somatotrophic hormone in ultrathin sections of porcine pituita ry embedded in Spurr resin. The specific boron-labelling of secretory vesicles in somatotrophs was detected by electron spectroscopic imagin g and confirmed by conventional immunogold labelling run in parallel. In comparison with immunogold, boron-labelled F(ab)'-fragments showed higher tagging frequencies, as was expected; the small uncharged immun oreagents have an elongated shape and carry the antigen-combining stru cture and the detection tag at opposite ends, thus allowing for high s patial resolution in electron spectroscopic imaging.