Tissue distribution of polaprezinc in rats determined by the double tracer method

The tissue distribution of polaprezinc (an insoluble zinc complex of L-carnosine) in rats was studied by the double tracer method using [U-14C-histidine]-, 65Zn-polaprezinc. The 65Zn-radioactivity was measured with an auto-gamma counter, and the 14C containing 65Zn was converted to an absolute count according to the calibration curve for quenching with a liquid scintillation counter with the spill-over method. After the administration of 14C-, 65Zn-polaprezinc to rats, the excretion ratio and time courses in the tissues of the 14C-and 65Zn-radioactivity were different each other. We found that polaprezinc was metabolized as endogenous amino acid or zinc after dissociation in the body. The zinc concentration in plasma reached its maximum at 1 h and decreased slowly, returning to the endogenous level at 11 h after the administration of non-labeled polaprezinc. The concentrations of zinc in liver, kidney, testis, prostate, and cerebrum remained rather constant. The replacement ratios of 65Zn to zinc in the tissues at its maximum percentage were 40% in plasma, 16-20% in liver, kidney, blood, and prostate. The low replacement ratios in testis and cerebrum (2-3%) suggested that zinc uptakes in testis and brain were regulated by the blood-testis-barrier and blood-brain-barrier, respectively.