'Dynorphin' in plasma: enzymatic artifact and authentic immunoreactivity

The potent opioid peptide dynorphin (DYN) is found in posterior pituitary vasopressinergic neurones and in adrenal medullary cells suggesting that secretion into plasma is likely. We have developed a sensitive radioimmunoassay in order to study plasma DYN in man. It transpired that extraction prior to assay was essential since unextracted plasma caused gross and non-parallel inhibition of binding of tracer. Plasma extracted using leached silica glass ( Vycor ) caused inhibition of tracer binding which diluted in parallel to synthetic DYN suggesting the presence of substantial amounts of DYN-like immunoreactivity ( irDYN ) in plasma. Further investigation however demonstrated that this irDYN was artifactual and caused by enzymatic degradation of tracer. Although use of Seppak C18 cartridges resulted in reliable extraction of synthetic porcine DYN from acidified plasma, we have not detected irDYN in any plasma so far studied using this technique. However, extraction of non-acidified plasma using our antibody coupled to Sepharose CNBr-activated 4B followed by gel filtration chromatography demonstrated a single peak of irDYN of molecular size similar to DYN. These data suggested that a small amount of a DYN-like peptide does circulate in human plasma although this is not identical to porcine DYN(1-17). The implication of our results for the measurement of other similar peptides in plasma is discussed.