Regulation of mammalian S-Adenosylmethionine decarboxylase

Abstract

S-Adenosylmethionine decarboxylase is a key enzyme in the biosynthesis of polyamines that is the rate limiting step in the formation of spermidine and spermine. The activity of S-adenosylmethionine decarboxylase is known to be regulated negatively by these polyamines and positively by their precursor, putrescine. A specific antiserum to S-adenosylmethionine decarboxylase was raised by immunizing rabbits with the homogeneous enzyme purified from rat prostate and a specific radioimmunoassay for the protein was set up. Using this radioimmunoassay it was found that a number of inhibitors of other steps in the polyamine biosynthetic pathway lead to increases in the amount of S-adenosylmethionine decarboxylase protein. These changes were caused by both a decreased rate of degradation and an increased rate of synthesis of the protein. The increased synthesis was due to two factors; a rise in the amount of translatable mRNA and an enhanced translation efficiency. The mRNA content of the prostate was substantially increased by treatment for 3 days with α-difluoromethylornithine (2% in drinking water). The translation of mRNA for S-adenosylmethionine decarboxylase was studied using a polyamine-depleted reticulocyte lysate supplemented with mRNA from rat prostate and the antiserum to precipitate the proteins corresponding to S-adenosylmethionine decarboxylase. These studies indicated that the enzyme was synthesized as an inactive precursor of M r 37,000 which was converted to the enzyme sub-unit of M r 32,000. The conversion of the precursor to the active sub-unit in vitro was increased by putrescine. The precursor could also be detected by immunoblotting of extracts from prostates of rats depleted of putrescine by treatment with the ornithine decarboxylase inhibitor, α-difluoromethylornithine. The translation of the S-adenosylmethionine decarboxylase mRNA in the reticulocyte lysates was strongly inhibited by the addition of spermidine or spermine demonstrating that polyamines directly inhibit the synthesis of S-adenosylmethionine decarboxylase. cDNA clones corresponding to S-adenosylmethionine decarboxylase were isolated using prostatic mRNA from polysomes enriched in S-adenosylmethionine decarboxylase by immunopurification. The use of these probes showed that rat ventral prostate contains two S-adenosylmethionine decarboxylase mRNA species of approximately 3.4 and 2.1 kb which differ in the 3′ non-translated sequence. The sequence of these cDNAs will enable the amino acid sequence of the precursor to be obtained. This will provide evidence on the origin of the pyruvate prosthetic group of S-adenosylmethionine decarboxylase. The sequence should also aid in further studies of the mechanism by which polaymines influence the translation of this mRNA.