Library of sequence-specific radioimmunoassays for human chromogranin A

Background: Human chromogranin A (CgA) is an acidic protein widely expresse d in neuroendocrine tissue and tumors. The extensive tissue- and tumor-spec ific cleavages of CgA at basic cleavage sites produce multiple peptides. Methods: We have developed a library of RIAs specific for different epitope s, including the NH2 and COOH termini and three sequences adjacent to dibas ic sites in the remaining part of CgA. Results: The antisera raised against CgA(210-222) and CgA(340 -348) require d a free NH2 terminus for binding. All antisera displayed high titers, high indexes of heterogeneity (similar to 1.0), and high binding affinities (K- eff(0) similar to 0.1 x 10(12) to 1.0 x 10(12) L/mol), implying that the RI As were monospecific and sensitive. The concentration of CgA in different t issues varied with the assay used. Hence, in a carcinoid tumor the concentr ation varied from 0.5 to 34.0 nmol/g tissue depending on the specificity of the CgA assay. The lowest concentration in all tumors was measured with th e assay specific for the NH2 terminus of CgA. This is consistent with the r elatively low concentrations measured in plasma from carcinoid tumor patien ts by the N-terminal assay, whereas the assays using antisera raised agains t CgA(210-222) and CgA(340-348) measured increased concentrations. Conclusion: Only some CgA assays appear useful for diagnosis of neuroendocr ine tumors, but the entire library is valuable for studies of the expressio n and processing of human CgA. (C) 1999 American Association for Clinical C hemistry.