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

The present study was designed to assess the feasibility of using smal l 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 s izes. At the beginning of the 10 d metabolic balance, two adults, cons uming their habitual diet, were given an intravenous injection of Zn-7 0. There was a fourfold difference in the administered dose between th e two subjects (0-445 and 2.078 mg). Blood samples were taken at regul ar intervals and plasma enrichment with Zn-70 measured by thermal ioni zation mass spectrometry. Urine and faeces were collected and analysed for Zn and Zn-70. Kinetic analysis of the plasma Zn-70 decay by sever al different methods was undertaken. It was apparent from both deconvo lution analysis of the short-term (0-90 min) decay data and four-compa rtment modelling of the longer-term (0-24 h) data that isotopic Zn ver y rapidly equilibrates with the plasma Zn and with a rapidly exchangin g non-plasma pool, probably located within the liver. This latter pool appears to contain less than 10 mg Zn and the peak of isotope enrichm ent occurs at about 20 min post injection. The later decay of plasma Z n enrichment appears to be dictated by exchange with a much larger poo l of approximate size 350 mg.