A reagent made of zinc sulphate (0-08 M) in a 0-4 M sodium salicylate solution at pH 7-3 precipitated most of the IgG when a small volume of human serum was added. Sera with normal IgG levels or polyclonal hyperglobulinaemia showed a close correlation between total IgG and zinc-precipitated IgG (r = + 0-95). In clinical material, not including IgG myeloma, zinc-soluble IgG varied between 0 and 6 mg/ml and was independent of the IgG serum concentration. In 31 normal subjects the average IgG concentration, as determined by the Technicon immunonephelometric method, was 10-2 +/- 1-7 mg/ml for total IgG and 2-2 +/- 1-0 mg/ml for the soluble fraction. Among 173 sera, including 24 from cord blood, 16 from pregnant women, and 133 from patients with miscellaneous diseases, no pathological conditions except three cases of IgG myeloma were found with a zinc-soluble IgG definitely above the normal values; zinc-soluble IgG levels were often low in patients with hyperglobulinaemia, and the difference was highly significant in liver disease. kappa and gamma light chains as well as the four IgG-Hp chain subclasses were found in both zinc-soluble fractions of normal IgG. A study of myeloma monoclonal IgG showed that globulins of classes 1, 3, and 4 could be either soluble or insoluble in the zinc reagent. One, G2, was mainly insoluble. Hexose and antistreptolysin contents per milligram normal IgG were not significantly different in either fraction. It is suggested that zinc-soluble IgG consists of the recently synthesized molecules, the zinc-solubility of which has not yet been decreased by protein association, lipid interaction, antigen binding, or enzymatic denaturation. Within this hypothesis, a low level of soluble IgG would mean either an increased precatabolic protein or a decreased synthesis.
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