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

Abstract

1. The production of 14CO2 from S-adenosyl[carboxyl-14C]methionine by rat liver extracts was investigated. It was found that, in addition to the well-known cytosolic putrescine-activated S-adenosylmethionine decarboxylase, 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% (v/v) Triton X-100 [confirming the report of Sturman (1976) Biochim. Biophys. Acta428, 56–69]. 2. 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. Instead, activity a 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, whereas the Triton-solubilized activity was strongly inhibited. 3. By using an acetone precipitate of Triton-treated homogenates as a source of the activity, it was found that decarboxylated S-adenosylmethionine was not present among the products of the reaction, although 5′-methylthioadenosine and 5-methylthioribose were found. Such extracts were able to produce 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 pH6 at 100°C for 30min (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. 4. These results indicate that the Triton-solubilized activity is not a real S-adenosylmethionine decarboxylase, but that 14CO2 is produced via a series of reactions involving degradation of the S-adenosyl-[carboxyl-14C]methionine. It is probable that this degradation can occur via several pathways. Our results would suggest that part of the reaction occurs via the production of S-adenosylhomocysteine, which can then be converted into 2-oxobutyrate via the transsulphuration pathway, and that part occurs via the production of homoserine by an enzyme converting S-adenosylmethionine into 5′-methylthioadenosine and homoserine lactone.