Enzymatic N-Acetylation of Lysine Analogs

Spermidine/spermine-N1-acetyltransferase-2 (SSAT2) acetylates thialysine and is not involved in polyamine metabolism

Spermidine/spermine-N1-acetyltransferase (SSAT1) is a short-lived polyamine catabolic enzyme inducible by polyamines and polyamine analogues. Induction of SSAT1 plays an important role in polyamine homoeostasis, since the N1-acetylated polyamines can be excreted or oxidized by acetylpolyamine oxidase. We have purified a recombinant human acetyltransferase (SSAT2) that shares 45% identity and 61% homology with human SSAT1, but is only distally related to other known members of the GNAT (GCN5-related N-acetyltransferase) family. Like SSAT1, SSAT2 is widely expressed, but did not turn over rapidly, and levels were unaffected by treatments with polyamine analogues. Despite similarity in sequence to SSAT1, polyamines were found to be poor substrates of purified SSAT2, having K(m) values in the low millimolar range and kcat values of <0.01 s(-1). The kcat/K(m) values for spermine and spermidine for SSAT2 were <0.0003% those of SSAT1. Expression of SSAT2 in NIH-3T3 cells was not detrimental to growth, and did not reduce polyamine content or increase acetylpolyamines. These results indicate that SSAT2 is not a polyamine catabolic enzyme, and that polyamines are unlikely to be its natural intracellular substrates. A promising candidate for the physiological substrate of SSAT2 is thialysine [S-(2-aminoethyl)-L-cysteine], which is acetylated predominantly at the epsilon-amino group with K(m) and kcat values of 290 muM and 5.2 s(-1). Thialysine is a naturally occurring modified amino acid that can undergo metabolism to form cyclic ketimine derivatives found in the brain and as urinary metabolites, which can undergo further reaction to form antioxidants. SSAT2 should be renamed 'thialysine N(epsilon)-acetyltransferase', and may regulate this pathway.

A novel member of the GCN5-related N-acetyltransferase superfamily from Caenorhabditis elegans preferentially catalyses the N-acetylation of thialysine [S-(2-aminoethyl)-L-cysteine]

The putative diamine N-acetyltransferase D2023.4 has been cloned from the model nematode Caenorhabditis elegans. The 483 bp open reading frame of the cDNA encodes a deduced polypeptide of 18.6 kDa. Accordingly, the recombinantly expressed His6-tagged protein forms an enzymically active homodimer with a molecular mass of approx. 44000 Da. The protein belongs to the GNAT (GCN5-related N-acetyltransferase) superfamily, and its amino acid sequence exhibits considerable similarity to mammalian spermidine/spermine-N1-acetyltransferases. However, neither the polyamines spermidine and spermine nor the diamines putrescine and cadaverine were efficiently acetylated by the protein. The smaller diamines diaminopropane and ethylenediamine, as well as L-lysine, represent better substrates, but, surprisingly, the enzyme most efficiently catalyses the N-acetylation of amino acids analogous with L-lysine. As determined by the k(cat)/K(m) values, the C. elegans N-acetyltransferase prefers thialysine [S-(2-aminoethyl)-L-cysteine], followed by O-(2-aminoethyl)-L-serine and S-(2-aminoethyl)-D,L-homocysteine. Reversed-phase HPLC and mass spectrometric analyses revealed that N-acetylation of L-lysine and L-thialysine occurs exclusively at the amino moiety of the side chain. Remarkably, heterologous expression of C. elegans N-acetyltransferase D2023.4 in Escherichia coli, which does not possess a homologous gene, results in a pronounced resistance against the anti-metabolite thialysine. Furthermore, C. elegans N-acetyltransferase D2023.4 exhibits the highest homology with a number of GNATs found in numerous genomes from bacteria to mammals that have not been biochemically characterized so far, suggesting a novel group of GNAT enzymes closely related to spermidine/spermine-N1-acetyltransferase, but with a distinct substrate specificity. Taken together, we propose to name the enzyme 'thialysine N(epsilon)-acetyltransferase'.

omega-Oxidation of fatty acids and the acetylation p-aminobenzoic acid

p-Aminobenzoic acid was fed to normal and alloxan-induced diabetic rats injected with [omega-14C]labeled and [2-14C]labeled fatty acids. The p-acetamidobenzoic acid that was excreted was hydrolyzed to yield acetate which was degraded. The distribution of 14C in the acetates formed when an [omega-14C]labeled fatty acid was injected was similar to that when a [2-14C]labeled fatty acid was injected. This contrasts with the finding that in acetates from 2-acetamido-4-phenylbutyric acid excreted when 2-amino-4-phenylbutyric acid was fed, there was a difference in the distributions of 14C, a difference attributable to omega-oxidation of the fatty acid. Acetylation of p-aminobenzoic acid is then concluded to occur in a different cellular environment than that of 2-amino-4-phenylbutyric acid, one in which omega-oxidation is not functional. When 2-amino-4-phenylbutyric acid was fed and [6-14C]palmitic acid injected, rather than [16-14C]palmitic acid, the distribution of 14C in acetate was the same as when [2-14C]palmitic acid was injected. This indicates that the dicarboxylic acid formed on omega-oxidation of palmitic acid does not undergo beta-oxidation to form succinyl-CoA. Thus, glucose is not formed via omega-oxidation of long-chain fatty acid.

[Cadaverine is an intermediate in the biosynthesis of arthrobactin and ferrioxamine E (author's transl)]

Cadaverin was more readily incorporated than lysine into arthrobactin from Arthrobacter pascens and into ferrioxamin E from Streptomyces glaucescens. From a racemic mixture only the L-isomer of lysine is incorporated. The L-lysine decarboxylase activity was measured in vivo and in vitro. The enzyme from Arthrobacter pascens is not inducable by lysine and completely repressed by 5.10(-6) M Fe3+. In Klebsiella pneumoniae, the producer of aerobactin, only a very low activity of L-lysine decarboxylase was detected.

Selenalysine and protein synthesis

Selenalysine is a lysine analog having the gamma-methylene group substituted by a selenium atom. It has been demonstrated that selenalysine is activated and transferred to tRNAlys by either Escherichia coli or rat liver aminoacyl-tRNA synthetases, and inhibits lysine incorporation into polypeptides in protein-synthesizing systems from E. coli, rat liver or rabbit reticulocytes. All tests were performed in comparison with thialysine, a lysine analog having the gamma-methylene group substituted by a sulfur atom. In all the reactions studied, both thialysine and selenalysine act as competitive inhibitors of lysine. With respect to thialysine, selenalysine act as competitive inhibitors of lysine. With respect to thialysine, selenalysine shows a slightly lower activity as lysine inhibitor.