Heterogeneity of immunoreactive calcitonin in human milk

Immunoreactive calcitonin (ICT) has been detected in human milk by radioimmunoassay (RIA), using antibodies raised against synthetic human calcitonin (hCT). The level of ICT was 10-40 times higher than the reference level of human serum. Gel chromatography of human milk disclosed a molecular weight heterogeneity of ICT, with at least two forms larger than the monomer (Mr greater than 70000 and 30000 approximately). The 30000 peak constituted the major fraction of ICT. No detectable amount of ICT was coeluted with [125I]CT. Reduction with disulphide cleaving agents, denaturation or acidification did not alter the gel chromatographic properties of ICT, while tryptic digestion caused partial degradation of ICT and formation of new ICT molecules with an approximate molecular weight of 6000.