Nickel(II)-binding constituents of human blood serum

Studies were undertaken to investigate the Ni(II)-binding properties of human blood serum and to identify the low-molecular-weight Ni(II)-binding constitutents in the serum. Three Ni(II)-binding fractions were obtained when labeled nickel chloride (63NiCl2) was added to the native serum. Of the total Ni(II), 95.7% was associated with albumin, 4.2% was bound to low-molecular-weight components, and a small fraction, usually less than 0.1% was associated with a high-molecular-weight protein that was eluted in the void volume of Sephadex G-150. Amino acids were shown to be responsible for the low-molecular-weight Ni(II)-binding fraction and L-histidine was found to be the main (Ni(II)-binding amino acid in human blood serum. Compared with albumin, L-histidine was shown to possess a greater affinity for Ni(II). Ni(II)-binding to human albumin became evident only when no more L-histidine was available. Since the concentration of albumin is much higher than the concentration of L-histidine in normal serum, most of the added Ni(II) was associated with albumin. The equilibria between Ni(II)-L-histidine and Ni(II)-albumin may facilitate the transport of Ni(ii) between blood and tissues.