Selenium
Determination in Human Serum Lipoprotein
Fractions by Neutron Activation Analysis
Attempts to explain the complicated nutritional and metabolic interrelationships of selenium and vitamin E (1) led to suggestions that both are associated with the lipoproteins of blood during transport ( 24). In this study, we have used a neutron activation method to measure the selenium content of normal human blood plasma lipoprotein fractions. Neutron activation techniques for selenium have previously been described by several investigators (cf. 5). The method has the sensitivity required for the very small amounts of selenium present, and the advantage over the isotope dilution technique with SeT5 that analyses indicate equilibrium conditions. Previous reports of the selenium content of blood lipoproteins have been published by McConnell and Levy (6), Hirooka and Galambos (7), and Roffler et al. ( 8).
Sample Preparation. Blood samples were obtained from normal adults. About 120 ml blood was obtained from each subject; 36 or 42 ml of serum was used for lipoprotein preparation and 6 ml of serum was retained. The lipoprotein fractions were prepared by differential ultracentrifugation according to the method of Lindgren et al. (9,10), and designated as follows: VLDL (very low-density lipoproteins, d < 1.006 gm/ml) ; LDL (low-density or /?-lipoprotein, d 1.006-1.036 gm/ml) ; and HDL (high-density or a-lipoprotein, d 1.063-1.21 gm/ml) (10). All samples were stored under nitrogen at 0ยฐC in glass vials as soon as they were collected.
Selenium Analyses. 2 ml samples of serum or lipoprotein were lyophilized and transferred to 10 mm quartz tubing sealed at one end. The other end was then carefully sealed to avoid charring. Aliquots of standard solutions of sodium selenite and glass-distilled water were dried in similar quartz tubes, and sealed. A sample (7.90 mg) of pure selenium metal was included for peak energy identification.
Weighed iron wires were wound around the quartz tubes so that the differences in neutron flux from sample to sample could be determined. The sample tubes were wrapped in aluminum foil and packed into aluminum reactor containers, which were sealed. The containers were 598