High molecular weight vasopressin: detection of a large amount in the plasma of a patient.

Using a sensitive and specific radioimmunoassay for arginine-vasopressin, we have searched for the presence of high molecular weight (HMW) vasopressin in the plasma of a patient with the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and an oat cell carcinoma of the lung. After incubation in 8 mol/l urea, one millilitre of the plasma from this patient was fractionated on a Sephadex G-50 column and the immunoreactive vasopressin content was evaluated before and after trypsin treatment of the eluted fractions. A wide peak of apparent mol. wt. 2500-6000 daltons was revealed only after tryptic digestion of each fraction. This peak contained the equivalent of 1900 pg of vasopressin. A tryptic digest of this peak, rechromatographed on Sephadex G-25, gave two small peptides the major one eluting at a position identical to vasopressin. These results demonstrate the presence of a large amount of HMW vasopressin in the plasma of a patient with SIADH and oat cell carcinoma of the lung.