Se 75-selenomethionine incorporation into human plasma proteins and erythrocytes

Abstract

The disappearance of Se 75-selenomethionine from blood following its i.v. injection and its reappearance in total plasma proteins, albumin, fibrinogen, and hemoglobin has been studied over 25 days in 2 groups of subjects: (1) a group of 8 normal volunteers, (2) a group of patients on whom parathyroid scanning with Se 75-selenomethionine was performed. These patients were premedicated with triiodothyronine for 4 days before the isotope administration. The free selenomethionine disappears rapidly from blood. By the tenth hour, virtually very little cold TCA soluble radioactivity could be detected. (1) There is gradual build-up of plasma protein bound radioactivity, reaching a maximum in 8 hours in the first group, and in 6 hours in the second group. It could be shown that a small fraction of the TCA insoluble fraction is bound to protein through a reducible seleno-sulfur bond. This reducible fraction practically disappears after the twelfth hour. The plasma protein activity at its maximum total value accounts for 14 to 22 per cent of the injected isotope in normal subjects, and 21 to 32 per cent in T3-treated patients. (2) The fall of the total plasma protein specific activity over the first 25 days followed a multiexponential pattern. The albumin specific activity build-up and decay curves could be defined. As with I 131 labeled albumin, a multiexponential disappearance curve could be demonstrated. The terminal slope was, however, less steep with the Se 75-selenomethionine label than has been shown by using an I 131 label. The mean T 1 2 was 21.8 days. The fibrinogen specific activity build-up and decay curves were defined. The disappearance curve could be fitted to a single exponential with a mean T 1 2 of 7.0 days. Triiodothyronine pretreatment markedly increased the uptake of radioactivity into this component. (3) The incorporation of Se 75-selenomethionine into hemoglobin was demonstrated to follow a pattern which strongly suggests that the label was incorporated into red cells during their early development. Ninty-five per cent of the red cell activity could be accounted for by Hb; 0.5 per cent of the red cell activity was associated with the red cell membrane. In 1 normal subject, the Hb specific activity dropped between the 100th and the 140th day to low levels. It is felt, therefore, that this represents the life span distribution range of normal erythrocytes.