ABSTRACT
Homology modeling and inhibitory studies using substrate analogs were undertaken to construct a possible three-dimensional structure, including the putrescine-binding site, of rat spermidine synthase based on its primary sequence. Of the ten cysteine residues of the enzyme, six residues were chemically determined as sulfhydryl; similarly, one residue (C25) was determined as the disulfide. Using the model obtained from the Swiss-Model protein-modeling server, and based on the crystal structure of the Thermotoga maritima enzyme, the three remaining residues were assigned as sulfhydryl. Discussions are presented on the counterpart of the C25 residue, based on the apparent role of the bacterial N-terminal peptide region in reinforcing the binding between protomers in a functional oligomeric form. The active sites of the bacterial and mammalian versions of the enzyme were very similar. The putrescine-binding site of the rat enzyme was investigated using IC(50) values of the analogs of two known potent inhibitors, n-butylamine and trans-4-methylcyclohexylamine (4MCHA). Our results indicated that 5-amino-1-pentene and 4MCHA possess comparable inhibitory activities towards the enzyme.
Subject(s)
Cysteine/chemistry , Putrescine/metabolism , Spermidine Synthase/chemistry , Amino Acid Sequence , Animals , Binding Sites/drug effects , Butylamines/chemistry , Butylamines/pharmacology , Cyclohexylamines/chemistry , Cyclohexylamines/pharmacology , Male , Models, Molecular , Molecular Sequence Data , Protein Conformation , Rats , Rats, Sprague-Dawley , Spermidine Synthase/antagonists & inhibitors , Spermidine Synthase/metabolismABSTRACT
Synthetic decarboxylated S-adenosyl-L-methionine (dcAdoMet), a mixture of the absolute configuration of S and R at the sulfonium center, was evaluated as a substrate for the measurement of spermidine synthase activity. The diastereomers were separated by HPLC with an isocratic elution, and the constant for racemization at the sulfur was determined to be 2.4x10(-6) s(-1) at 37 degrees C and pH 1.5 for the first-eluted biologically active isomer (S-dcAdoMet) and 2.0x10(-6) s(-1) for the second-eluted biologically inactive isomer (R-dcAdoMet). The peak area ratio of S-dcAdoMet to R-dcAdoMet of 48 to 52 in HPLC supported the different racemization constants. Similar substrate activity of dcAdoMet to that of S-dcAdoMet was demonstrated by enzymatic spermidine synthesis. It was shown from the result that the racemized [methyl-(14)C]dcAdoMet prepared in this report was useful for measuring spermidine synthase activity.
