Chemiluminescence immunoassay for somatomedin C in serum

To select the best tracer for use in a competitive immunoassay, we conjugated human somatomedin C (SmC) to various chemiluminescent compounds via two different synthetic pathways. Naphthylhydrazides and arylhydrazides, used as the labels, were incorporated via their imidate or their succinimide esters. Conjugating the carboxy terminal of (amino ethyl)ethyl-isoluminol to SmC via a succinimide linkage supplied the most sensitive detection limit and the most immunoreactive conjugate. We developed an immunoassay based on the use of this conjugate, and evaluated dextran-coated charcoal, second-antibody precipitation, and solid-phase immunoprecipitation for separating bound and free label. This chemiluminescent method has a detection limit of 16 pg per tube, and it is accurate and precise. Correlation studies with a conventional radioimmunoassay (x) for SmC gave the following regression equation: y = 0.66x + 3.76 (r = 0.953, n = 30); the slight discrepancies between the two methods are probably ascribable to the use of different antibodies. We thus propose this chemiluminescence immunoassay as an inexpensive and sensitive alternative to radioimmunoassay for measuring SmC in serum or in extracts of serum.