Role of disulfide interchange enzyme in immunoglobulin synthesis

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Abstract
The role of disulfide interchange enzyme (thiol:protein disulfide oxidoreductase) in protein biosynthesis was evaluated by studying the enzyme from mouse lymphoid tissue. The enzyme isolated from lymphoid cells had no tissue-specific characteristics. It was identical with the enzyme synthesized by mouse liver in its biochemical and immunological properties and its capacity to promote both disulfide bond formation and insulin degradation. In contrast to liver, the levels of enzyme in lymphoid tissues varied with Ig secretory activity. Assays of lymphoid cells and their transformed counterparts showed that the enzyme contents of cells actively secreting Ig were 1-2 orders of magnitude higher than that of unstimulated B cells or non-Ig-producing T cells. The increase in enzyme levels paralleled the increase in Ig synthesis after antigen or mitogen stimulation and was independent of the class of Ig produced. This correlation indicated that the enzyme plays a critical role in the formation of intramonomer bonds common to all Ig molecules. Supporting data were obtained by assaying the ability of the enzyme to promote the polymerization of mouse pentamer IgM in vitro. The enzyme catalyzed the formation of the interchain bonds required for monomer IgM assembly, but not the formation of the intermonomer bonds required for pentamer assembly. The sum of these results provides strong evidence that disulfide interchange enzyme functions in the in vivo synthesis of protein disulfide bonds.