Characterization of somatostatin-like components in the tumors and plasma of a patient with a somatostatinoma

Somatostatin-like immunoreactive polypeptides with approximate molecular weights of 1, 500–2, 000 (peak I), 2, 500–3, 500 (peak II), and 10, 000–15, 000 (peak III) were isolated from the peripheral plasma and from extracts of tumor tissue of a patient with a pancreatic somatostatinoma and hepatic metastases. The peak I component had approximately the same molecular charge as somatostatin, but the peak II component was appreciably more basic. The peak III component was heterogeneous with respect to charge and dissociated into smaller immunoreactive polypeptides during isoelectric focusing or after treatment with strongly denaturing solvents. In plasma, the levels of the larger molecular weight components (peak II and peak III), relative to that of the somatostatin-size component (peak I), were higher than in tissue extracts, suggesting a lower rate of clearance of the larger polypeptides from the circulation. In response to a nutrient stimulus, the peak I component increased to a greater extent than the larger components. Plasma levels of larger immunoreactive polypeptides relative to the peak I component in patient's plasma were higher after complete resection of the pancreatic tumor than preoperatively, suggesting that the rate of release of these putative precursor forms from the metastases was greater than from the primary tumor.