The tissue distribution of polaprezinc (an insoluble zinc complex of L-carn
osine) in rats was studied by the double tracer method using [U-C-14-histid
ine]-, Zn-65-polaprezinc. The Zn-65-radioactivity was measured with an auto
-gamma counter, and the C-14 containing Zn-65 was converted to an absolute
count according to the calibration curve for quenching with a liquid scinti
llation counter with the spill-over method. After the administration of C-1
4-, Zn-65-po- laprezinc to rats, the excretion ratio and time courses in th
e tissues of the C-14-and Zn-65-radioactivity were different each other. We
found that polaprezinc was metabolized as endogenous amino acid or zinc af
ter dissociation in the body. The zinc concentration in plasma reached its
maximum at 1 h and decreased slowly, returning to the endogenous level at 1
1 h after the administration of non-labeled polaprezinc. The concentrations
of zinc in liver, kidney, testis, prostate, and cerebrum remained rather c
onstant. The replacement ratios of Zn-65 to zinc in the tissues at its maxi
mum percentage were 40% in plasma, 16-20% in liver, kidney, blood, and pros
tate. The low replacement ratios in testis and cerebrum (2-3%) suggested th
at zinc uptakes in testis and brain were regulated by the blood-testis-barr
ier and blood-brain-barrier, respectively. (C) 1999 Elsevier Science B.V. A
ll rights reserved.