The rates at which terbium (III) ion is removed from the two metal binding sites of human serum transferring by tartaric acid have been evaluated. Solutions are buffered at pH 7.4 by 0.01 mol/L N-2-hydroxy- ethylpiperazine-N'-2-ethanesulfonic acid (Hepes) and maintained at room temperature. According to their kinetic behaviors, it is proposed that there are two parallel removal pathways. One is saturation pathway, the other is first-order pathway. The results show that terbium(III) ion removal is through both pathways for the C-terminal site, while through simple first-order pathway for the N-terminal site. The addition of sodium chloride to the buffer accelerates terbium (III) ion removal from both forms of monoterbium transferring, and the C- terminal monoterbium transferrin is more easily affected by sodium chloride.

Key words: PROTEIN, TERBIUM, FLUOROSPECTROPHOTOMETRY, KINETIC STUDY, TARTARIC ACID, SODIUM CHLORIDE