Intracellular labeling with ferritin conjugates. A specificity problem due to the affinity of unconjugated ferritin for selected intracellular sites.

Nonspecific binding of ferritin to chromatin and the cytoplasmic aspect of the nuclear envelope was observed when nonantigenic, serum-washed hepatocyte nuclei were incubated in ferritin-antibody conjugates. This labeling was duplicated when nuclei from a wide range of species and cell types were exposed to unconjugated ferritin. Unconjugated ferritin binding to nuclei did not depend on a subpopulation of denatured molecules or on the ferritin purification procedure. Binding occurred equally on unfixed and formaldehyde-fixed nuclei, but no ferritin bound to glutaraldehyde-fixed nuclei. Inconjugated ferritin also bound to the cytoplasmic aspects of the rough endoplasmic reticulum and the plasma membrane. The tracer did not bind to lysosomes, mitochondria, Golgi vesicles, the extracellular surface of plasma membranes, or the intracisternal surfaces of ruptured nuclear envelopes. The addition of 0.4 M KCl or 0.7 M NaCl to ferritin solutions and washing media at neutral pH reduced the binding of conjugated and unconjugated ferritin to nuclei to about 3% of that seen in 0.10 M phosphate buffer alone. The added salts caused little extraction of nuclear contents from formaldehyde-fixed nuclei. The use of one of these salts in ferritin conjugates should considerably improve the specificity of intracellular labeling.