Transamination of some sulphur- or selenium-containing amino acids by bovine liver glutamine transaminase.

S-(3-aminopropyl)cysteine and Se-(3-aminopropyl)selenocysteine are deaminated by bovine liver glutamine transaminase. The corresponding alpha-keto acids, S-(3-aminopropyl)-thiopyruvic acid and Se-(3-aminopropyl)selenopyruvic acid, are produced which spontaneously cyclize to ketimine derivatives. They have been identified by comparing their UV absorption spectra and some chemical or chromatographic properties with chemically synthesized authentic samples. Also S-(2-aminoethyl)homocysteine is the substrate for the enzyme. Kinetic parameters determined in comparison to thialysine and selenalysine show that neither the presence of a sulphur or a selenium atom nor the relative position of the atom in the carbon chain appreciably affects the substrate specificity of the enzyme. However, the length of the carbon chain has some influence on it.

Aspartokinase III repression and lysine analogs utilization for protein synthesis.

The extents of thialysine and selenalysine incorporation into cell proteins were compared in E. coli KL16 and in a mutant able to grow equally well in the presence or in the absence of both lysine analogs. The mutant differs from the parental strain in the repression of aspartokinase III (AKIII), the first enzyme of the lysine biosynthetic pathway. No analog incorporation into proteins was observed in mutant cells grown in the presence of either analog, whereas a marked analog incorporation was observed in the parental strain, where up to 17% and 12% of protein lysine can be substituted by thialysine and selenalysine respectively. In the parental strain grown in media containing either analog at different concentration the extent of analog incorporation into proteins is related to the extent of AKIII repression.

Selenalysine transamination by a bovine brain enzyme.

Selenalysine is deaminated by glutamine transaminase from bovine brain, leading to the production of the corresponding alpha-ketoacid, which spontaneously cyclizes to a ketimine form. Selenalysine shows a good affinity for the enzyme.

Studies on recognition of selenahomolysine by aminoacid transport systems and aminocyl-tRNA synthetase.

In E. coli, Se-3 aminopropylselenocysteine or selenahomolysine (SeHL) does not affect intracellular lysine transport, i.e. it cannot bind E. coli lysine transport systems. In CHO cells it inhibits cationic aminoacid transport system, but only in the presence of Na+, this indicating that it behaves like polar neutral aminoacids. On the other hand, it poorly affects leucine transport both in the presence and in the absence of Na+. SeHL is not activated by aminoacyl-tRNA synthetase preparations from bacterial and mammalian sources, thus it cannot be utilized for protein synthesis.

Oxidative deamination of Se-(1-carboxyethyl)-,Se-(1-carboxypropyl)- and Se-(2-carboxyethyl)-selenocysteine by snake venom L-aminoacid oxidase.

Details are reported for the synthesis of Se-(1-carboxyethyl)-selenocysteine (1-CESeC), Se-(1-carboxypropyl)-selenocysteine (1-CPSeC) and Se-(2-carboxyethyl)-selenocysteine (2-CESeC). They can be obtained in pure cristalline form with good yield. Some chromatographic properties, useful for their identification, are described. The three aminoacids are good substrates for snake venom L-aminoacid oxidase, giving the corresponding alpha-ketoacids as reaction products.

Intracellular transport of thialysine and selenalysine in CHO cells.

The intracellular transport of thialysine and selenalysine in CHO cells has been studied. Data have been obtained indicating that the two lysine analogs can be transported by both the cationic aminoacid transport system and by the L transport system. The affinity of the cationic aminoacid transport system is similar for the two lysine analogs but lower than that for lysine and the affinity of the L transport system for the two lysine analogs is lower than that for leucine.

Effects of selenalysine on thialysine resistant CHO cells.

A thialisyne resistant variant clone of CHO cells also shows a lower sensitivity to selenasyne, the lysine analog containing selenium. Growth rate, cell viability and protein synthesis rate are less affected by selenasyne in the variant compared to the parental strain. Data are reported showing that during cellular growth of either strain some toxic derivatives of selenasyne are produced and accumulated in the culture medium even in the presence of excess lysine.

Effects of selenalysine on CHO cells.

Selenalysine, the lysine isolog with the 4-methylene group substituted by a Selenium atom, inhibits growth rate and plating efficiency of Chinese Hamster Ovary (CHO) cells. It does not affect DNA and RNA synthesis, but inhibits protein synthesis. Cells grown in the presence of selenalysine show a reduced viability and an increased cell volume. Almost all the effects of selenalysine on CHO cells can be reversed by lysine, thus indicating that selenalysine acts mainly in competition with lysine by impairing its utilization.

Thialysine and selenalysine utilization for protein synthesis by E. coli in comparison with lysine.

Utilization of thialysine and selenalysine for protein synthesis by a lysine requiring E. coli mutant was studied. Incorporation into proteins of thialysine or selenalysine, added to culture medium together with lysine, becomes evident when the amount of available lysine in the medium is highly reduced, that is the mutant utilizes the isologs only after all the available natural aminoacid has been utilized. Compared to selenalysine, thialysine is better utilized; when both isologs are present in the medium at equal concentrations, up to 46% of protein lysine is substituted by thialysine and only 12% by selenalysine.

Selenalysine utilization for growth and protein synthesis by a lysine requiring E. coli mutant.

Selenalysine can be utilized in substitution of lysine by a lysine requiring E. coli mutant. The presence of some lysine in the culture medium is necessary to allow selenalysine utilization for growth; in the presence of an excess of lysine, selenalysine is not utilized. When utilized, selenalysine gives rise to an increase of final growth. However, it shows some toxic effects as demonstrated by the decrease of both growth rate and cell viability. Selenalysine is incorporated into proteins in substitution of lysine. Up to a maximum of 50% of total protein lysine can be substituted. The decrease of cell viability is correlated with the extent of lysine substitution.