The decarboxylation of S-adenosylmethionine by detergent-treated extracts of rat liver

Abstract

The production of 14CO2 from S-adenosyl[carboxyl-14C]methionine by rat liver extracts was investigated. Cytosolic putrescine-activated S-adenosylmethionine decarboxylase and an activity carrying out the production of 14CO2 could be extracted from a latent, particulate or membrane-bound form by treatment with buffer containing 1% (vol/vol) Triton X-100. The formation of 14CO2 by such detergent-solubilized extracts differed from that by cytosolic S-adenosylmethionine decarboxylase in a number of ways. The reaction by the solubilized extracts did not require putrescine and was not directly proportional to time of incubation or the amount of protein added. Activity showed a distinct lag period and was much greater when high concentrations of the extracts were used. The cytosolic S-adenosylmethionine decarboxylase was activated by putrescine, showed strict proportionality to protein added and the reaction proceeded at a constant rate. Cytosolic activity was not inhibited by homoserine or by S-adenosylhomocysteine but the Triton-solubilized activity was strongly inhibited. Using an acetone precipitate of Triton-treated homogenates as a source of the activity, decarboxylated S-adenosylmethionine was not present among the products of the reaction; 5''-methylthioadenosine and 5-methylthioribose were found. Such extracts produced 14CO2 when incubated with [U-14C]-homoserine and 14CO2 production was greater when S-adenosyl[carboxyl-14C]methionine that had been degraded by heating at pH 6 at 100.degree. C for 30 min (a procedure known to produce mainly 5''-methylthioadenosine and homoserine lactone) was used as a substrate than when S-adenosyl[carboxyl-14C]methionine was used. The Triton-solubilized activity is apparently not a real S-adenosylmethionine decarboxylase but 14CO2 is produced via a series of reactions involving degradation of the S-adenosyl-[carboxyl-14C]methionine. This degradation can probably occur via several pathways. Part of the reaction apparently occurs via the production of S-adenosylhomocysteine, which can then be converted into 2-oxobutyrate via the transsulpfuration pathway and part occurs via the production of homoserine by an enzyme converting S-adenosylmethionine into 5''-methylthioadenosine and homoserine lactone.