title: Studies of the specificity and kinetics of rat liver spermidine/spermine N1-acetyltransferase.
abstract: The substrate specificity and kinetic mechanism of spermidine N1-acetyltransferase from rat liver was investigated using a highly purified (18 000-fold) preparation from the livers of rats in which the enzyme was induced by treatment with carbon tetrachloride (1.5 ml/kg body wt. 6h before death). The enzyme catalysed the acetylation of spermidine, spermine, sym-norspermidine, sym-norspermine, N-(3-aminopropyl)-cadaverine, N1-acetylspermine, 3,3'-diamino-N-methyldipropylamine and 1,3-diaminopropane, but was inactive with putrescine, cadaverine, sym-homospermidine and N1-acetylspermidine. These results suggest that the enzyme is highly specific for the acetylation of a primary amino group that is separated by a three-carbon aliphatic chain from another nitrogen atom (i.e. the substrates are of the type H2N[CH2]3NHR). The maximal rates of acetylation of 1,3-diaminopropane and 3,3'-diamino-N-methyldipropylamine were much lower than the maximal rates with spermidine or sym-norspermidine as substrates, suggesting a preference for a secondary amino group bearing the aminopropyl group that is acetylated. The best substrates for acetylation were sym-norspermidine and sym-norspermine, which had Km values of about 10 micrograms and Vmax. values of about 2 mumol of product/min per mg of enzyme compared with Km of 130 microM and Vmax. of 1.3 mumol/min per mg for spermidine. N1-Acetylspermidine (the product of the reaction) and N8-acetylspermidine were weak inhibitors and were competitive with spermidine, having Ki values of about 6.6 mM and 0.4 mM respectively. N1-Acetylspermidine was a non-competitive inhibitor with respect to acetyl-CoA. CoA was also inhibitory to the reaction, showing non-competitive kinetics when either [acetyl-CoA] or [spermidine] was varied. These results suggest that the reaction occurs via an ordered Bi Bi mechanism in which spermidine binds first and N1-acetyl-spermidine is the final product to be released.
mesh_terms: D000107:Acetylation; D000123:Acetyltransferases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D000818:Animals; D003065:Coenzyme A / Q000494:pharmacology; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D051381:Rats; D013095:Spermidine / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology; D013379:Substrate Specificity; D000099228:Diamine N-Acetyltransferase
chemical_list: D000123:Acetyltransferases; D003065:Coenzyme A; D013095:Spermidine; D000099228:Diamine N-Acetyltransferase; C030223:N(8)-acetylspermidine