Levels and molecular properties of secretoneurin-immunoreactivity in the serum and urine of control and neuroendocrine tumor patients

We have determined the levels of secretoneurin (SN), a novel 33-amino acid neuropeptide belonging to the class of chromogranins, in the serum and urine of healthy subjects and patients suffering from various tumors. SN serum levels averaged 22.1 ± 1.1 fmol/mL. They were 5-fold higher in younger children and then declined continuously. SN levels were positively correlated with serum creatinine, suggesting an influence of renal function on the clearance of SN from the serum. In the urine 80.0 fmol/mL SN was present. In patients with endocrine tumors like gut carcinoids, endocrine pancreatic tumors, oat cell lung carcinomas, and pheochromocytomas, SN serum levels were elevated up to 45-fold. Patients suffering from neuroblastomas, insulinomas, pituitary adenomas including acromegaly, and solid nonendocrine tumors had concentrations in the normal range. In human serum, SN-immunoreactivity was confined to the free peptide SN; neither larger intermediate-sized forms nor the precursor secretogranin II were detected. An efficient removal of the small molecule SN from the serum by the kidney explains why SN serum levels are lower when compared to chromogranin A, which is present as large molecule in serum.