Distribution of protein-bound zinc in normal and cirrhotic serum

Abstract

The distribution of ⁶⁵Zn among the serum proteins of 18 cirrhotic patients and 10 normal individuals was studied. Total zinc was measured in each serum. Zinc was bound to albumin, transferrin and <i>α</i>₂-macroglobulin in all subjects. It was bound to IgG in all cirrhotics and five of ten normals. Utilizing the percentage distribution of ⁶⁵Zn and total serum zinc, the amount bound in each fraction was calculated. Albumin-bound zinc correlated well with total serum zinc (r = 0.97). Total serum zinc was lower in cirrhotics than in normals (p < 0.01). The low serum zinc found in cirrhotics appears to be a function of serum albumin concentration. Zinc was consistently bound to transferrin and <i>α₂</i>-macroglobulin. The total amount bound to these proteins (Tf-<i>α₂</i>M) was of remarkably constant and equivalent size in normals and cirrhotics. Transferrin concentration was significantly lower (p < 0.01) in cirrhotics. The mean concentration of <i>α</i>₂-macroglobulin was higher in cirrhotics although the difference was not significant (p < 0.10). There appears to be a reciprocal relationship between the amount of zinc bound to transferrin and that bound to <i>α</i>₂-macroglobulin. However, the total amount bound to Tf-<i>α₂</i>M remains remarkably constant even when total serum zinc falls as low as 12 μg./ml. The constancy of the amount of zinc bound to Tf-<i>α₂</i>M implies metabolic control of this fraction. When serum zinc falls to very low levels, it may account for nearly one-half of the total serum zinc. It is suggested that transferrin and <i>α₂</i>-macroglobulin may have an important role in internal zinc exchange.