Abstract
Araldite sections of rat pituitary intermediate lobe were used with anti-17-39 adrenocorticotropin in the unlabeled antibody enzyme method to compare electron microscopic immunocytochemical staining by peroxidase-antiperoxidase complex (PAP) with antiserum or purified antibody to peroxidase followed by peroxidase (PO). Quantitation of normalized optical densities of secretory granules offered high significance comparison (P < 0.0001) of experimental with control values and of experimental values with each other. Use of purified antibody (prepared by a new density gradient method) yielded four times higher sensitivity than antiserum to PO, while a 6.5-fold increase would have been expected from the degree of contamination of anti-PO in the serum by nonanti-PO immunoglobulin. Use of PAP was four to five times more sensitive than purified anti-PO and 20 times more sensitive than antiserum to PO. The increased sensitivity of PAP is explained by the high over-all binding affinity of PO for anti-PO in the cyclic PAP molecule, thus preventing the losses of PO that occur during washing when anti-PO and PO have been applied in sequence. Identification of the characteristic, cyclic PAP molecules affords confirmation of specific staining at high resolution. In the sequential application of anti-PO and PO, no PAP molecules are formed, thus making distinction of specific from nonspecific deposition of enzyme reaction products ambiguous. With the use of anti-17-39 ACTH and the intermediate lobe, the unlabeled antibody enzyme method was 16,000-100,000 times more sensitive than radioimmunoassay.