THE MEASUREMENT OF EXCHANGEABLE POOLS OF ZINC USING THE STABLE-ISOTOPE ZN-70

The present study was designed to assess the feasibility of using small doses of a stable isotope of Zn to follow plasma kinetics over a 10 d period and, hence, make deductions about Zn turnover and body pool sizes. At the beginning of the 10 d metabolic balance, two adults, consuming their habitual diet, were given an intravenous injection of Zn-70. There was a fourfold difference in the administered dose between the two subjects (0-445 and 2.078 mg). Blood samples were taken at regular intervals and plasma enrichment with Zn-70 measured by thermal ionization mass spectrometry. Urine and faeces were collected and analysed for Zn and Zn-70. Kinetic analysis of the plasma Zn-70 decay by several different methods was undertaken. It was apparent from both deconvolution analysis of the short-term (0-90 min) decay data and four-compartment modelling of the longer-term (0-24 h) data that isotopic Zn very rapidly equilibrates with the plasma Zn and with a rapidly exchanging non-plasma pool, probably located within the liver. This latter pool appears to contain less than 10 mg Zn and the peak of isotope enrichment occurs at about 20 min post injection. The later decay of plasma Zn enrichment appears to be dictated by exchange with a much larger pool of approximate size 350 mg.