Measurement of 68Zn and 70Zn in human blood in reference to the study of zinc metabolism.

A method based on radiochemical neutron activation analysis has been developed for measurement of the stable isotopes 68Zn and 70Zn in human plasma and red cells. The method has been applied to measure the extent of 70Zn enrichment in plasma samples from four healthy adult volunteers who had each consumed a single 3.2 mg dose of 70ZnCl2 in fasting state. It is shown that measurements of 70Zn/68Zn ratio in plasma and red cell sample sizes of 2 ml can be made with precisions of 10% or less depending on the degree of enrichment achieved. The limiting factor in the precision of these measurements appears to be related to the counting statistics of the 386 kev photopeak (71mZn) which improves with increased enrichment. Clinical feasibility trials on these subjects with regard to plasma and red cell 70Zn enrichment have been carried out over a 24-h postadministration period. The results are consistent with kinetic studies reported in literature with radiozinc, and show that this method can be used to study kinetics of plasma appearance of 70Zn after oral administration of the isotope under physiological intake conditions. The present results further indicate that under these experimental conditions, enrichment of red cells can be measured after 24 h or longer postadministration periods, but short-term measurements yield marginal results. This new method provides an alternative approach to the use of radiolabeled zinc for the study of human zinc metabolism and appearance of isotopic zinc in plasma after oral administration under physiological intake conditions. It can be applied to all human population groups as a noninvasive and safe method which does not require safety considerations arising from human use of radiotracers. However, its application is both more expensive and limited in terms of sample throughput as compared with the radiotracer technique.