Determination, stable isotope enrichment and kinetics of direct-reacting copper in blood plasma

Direct-reacting Cu is defined as Cu2+ and other forms of Cu that readily exchange with Cu2+ in blood plasma. An analytical method was developed for 1) quantification of direct-reacting Cu by stable isotope dilution and 2) determination of Cu-65 enrichment of direct-reacting Cu in plasma samples from in vivo tracer studies. The method involved addition of enriched Cu-65 to plasma, extraction with sodium diethyldithiocarbamate in mineral oil, and analysis by inductively coupled plasma mass spectrometry. Optimum sodium diethyldithiocarbamate concentration for the extraction was 0.16 mM. Direct-reacting Cu (means +/- SD) varied from 3.4 +/- 0.5% of whole plasma Cu in dairy cows (n = 7) and 3.4 +/- 0.3% in healthy men (n = 10) to 16.6 +/- 3.7% in dogs (n = 3). After intravenous infusion of enriched Cu-65 into two healthy men, biological half-lives of 8.7 and 12.3 min were determined for direct-reacting Cu.