Membrane structure: The reactivity of tryptophan, tyrosine and lysine in proteins of the microsomal membrane

Abstract

1. 1. A microsomal subfraction devoid of adsorbed soluble proteins has been prepared from rat liver by density gradient centrifugation. Through the use of group-specific reagents the reactivity of tryptophanyl, tyrosyl, and lysyl side chains of the membrane has been examined. Significant differences are noted when the results are compared to literature values for non-membrane proteins. 2. 2. In the native microsomal membrane, tryptophan was unreactive toward N- bromosuccinimide . The results suggest that strong tryptophan interactions may exist which could likely have an important role in the maintenance of normal membrane structure. Furthermore, tryptophan appears to be involved at different levels of structural organization since extremely sharp transitions in reactivity were noted when sodium dodecyl sulfate was used as a perturbant. 3. 3. 85% of the tyrosyl residues occupied an exposed position on the membrane surface and were characterized by their ease of nitration in the absence of denaturing agents. The unusual accessibility of tyrosine suggests that side chain interactions involving tyrosyl residues do not play a prominent structural role. 4. 4. Carbamylation studies revealed that only 56% of the lysyl residues in the native membrane were available for reaction with cyanate while an additional 20–25% became reactive in the presence of urea. The remaining lysyl residues were unreactive under a wide variety of denaturing conditions. 5. 5. Sodium dodecyl sulfate interacted with the membrane in such a way as to markedly decrease the apparent accessibility of tyrosyl and lysyl residues while substantially increasing the reactivity of tryptophan.