Selenium Toxicity: Aminoacylation and Peptide Bond Formation with Selenomethionine

Abstract

Selenomethionine and methionine were compared as substrates for in vitro aminoacylation, ribosome binding, and peptide bond formation with preparations from wheat germ. Selenomethionine paralleled methionine in all steps of the translation process except peptide bond formation. Peptide bond formation with the initiating species of tRNAMet demonstrated that selenomethionyl-tRNAM't was less effective as a substrate than was methionyl-tRNAf't. Participation of selenomethionine in the initiation process of translation could be expected to reduce the overall rate of protein synthesis and might aid in explaining selenium toxicity in selenium-sensitive plants.methionine, each step enroute to incorporation of this amino acid into protein should be studied independently of the other. For this purpose, ribosomes and supernatant fractors, prepared from wheat germ, were used. Selenomethionine and methionine functioned as equivalent substrates in the aminoacylation reaction and in ribosome binding of the aminoacyl-tRNA. Peptide bond formation between puromycin and selenomethionine, in contrast, was curtailed compared to bond formation between puromycin and methionine. As an explanation for selenium toxicity at the subcellular level, it would appear that selenomethionine exerts one of its effects in wheat by limiting the rate at which the first peptide bond is formed during protein synthesis. MATERIALS AND METHODSMethionine is a key intermediate in several crucial metabolic reactions. Since methionine contains a sulfur atom, substances which disrupt sulfur metabolism, such as those that contain selenium, in all likelihood will interfere with the metabolism of this sulfur amino acid. In selenium-sensitive plants, the assimilation of selenium usually parallels that of sulfur. As a consequence, selenoanalogs of several sulfur-containing metabolites can be synthesized (17). One of these analogs, selenomethionine, is known to be formed by a wide variety of plants (7,9,(16)(17)(18). Although the biochemical characteristics of selenomethionine resemble those of methionine in a great many ways, the metabolic activity of selenomethionine apparently differs in certain respects; one of these differences is, in general, reaction rate (4,5,11,12). Examination of those reactions in which selenomethionine functions at a rate different than methionine could reveal some of the factors that underlie selenium toxicity.One vital metabolic process with which these methionine analogs might interfere is the biosynthesis of protein. Methionine, required as the initiating amino acid during the translation process, is also incorporated at internal positions of most protein chains. Alteration of initiation process of translation or with the chain elongation process could drastically affect normal functions of the cell. Previous in vitro studies with polysomes from Vigna radiata root tips have established that methionine is a slightly better substrate than is selenomethionine during chain elongation, provided that both amino acids are supplied s...

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