Binding and inactivation of Pitressin by serum and plasma in vitro were studied. The pressor activity of Pitressin added to human serum was usually lower than that of Pitressin dissolved in saline of the same concentration. Plasma did not reduce the activity under the experimental condition used. The reduction of activity seemed to be due to inactivation of the hormone progressing during a period of assay, not by binding of the hormone by serum protein. Rat serum samples tested did not affect the activity, while dog serum reduced it. The Pitressin binding capacity of serum, measured by dialysis technique, was approximately same as that of plasma. Both Pitressin binding and inactivating capacities differed from one human serum to another.
Human sera with larger binding capacity inactivated Pitressin to larger extent, and a significant correlation existed between these two capacities.
COHN fractions II+III and sometimes IV-4 attributed to both Pitressin binding and inactivation. A correlation between these capacities was also found in COHN fractions.