EBT, a tryptophan contaminant associated with eosinophilia myalgia syndrome, is incorporated into proteins during translation as an amino acid analog

Abstract

The tryptophan dimer 1,1'-ethylidenebis[L-tryptophan] was identified as a contaminant of tryptophan preparations associated with Eosinophilia-Myalgia Syndrome. In this paper, we describe experiments examining the hypothesis that 1,1'-ethylidenebis[L-tryptophan] acts as an amino acid analog replacing L-tryptophan during the synthesis of proteins. We propose further that proteins containing 1,1'-ethylidenebis[L-tryptophan] are rejected in an autoimmune process identified clinically as Eosinophilia-Myalgia Syndrome. Rabbit reticulocyte lysates containing an estimated 1 microM L-tryptophan were used to assay the ability of 1,1'-ethylidenebis[L-tryptophan] to compete with 3H-L-tryptophan for incorporation into proteins translated from BMV RNA. 1,1'-Ethylidenebis[L-tryptophan] in concentrations of 40, 80 and 110 microM reduced lysate 3H-L-tryptophan incorporation to 81%, 76% and 75% of control incorporation obtained in the absence of 1,1'-ethylidenebis[L-tryptophan]. In the presence of 20 microM L-tryptophan, 110 microM 1,1'-ethylidenebis[L-tryptophan] reduced 3H-L-tryptophan incorporation to 56% of control incorporation. In contrast, ethyl-L-tryptophan did not significantly reduce 3H-L-tryptophan incorporation. In the presence of 110 microM 1,1'-ethylidenebis[L-tryptophan] and 20 microM L-tryptophan, 3H-L-leucine incorporation was not significantly reduced compared to incorporation in the absence of 1,1'-ethylidenebis[L-tryptophan], demonstrating that proteins were translated to full length during elongation. These findings suggest that 1,1'-ethylidenebis[L-tryptophan], but not ethyl-L-tryptophan, reduced 3H-L-tryptophan incorporation into proteins by substituting for L-tryptophan rather than by causing premature termination or significant slowing of nascent protein chains.