S-adenosylmethionine and S-adenosylhomocysteine transitions in encysting Physarum flavicomum amoebae

Previous studies from this laboratory suggested that in Physarum flavicomum an S-adenosylmethionine (SAM) related metabolic flux is involved in the developmental control of microcyst formation. In the present study, this phenomenon was further analyzed by comparing the metabolic effects of compounds found to favor, stimulate, or inhibit the normal encystment process. The compounds utilized were adenine, cycloleucine, L-ethionine, DL-selenomethionine, cadaverine, cyclohexylammonium sulfate, papaverine, mycophenolic acid, decoyinine, putrescine, spermidine, and spermine. Following incubation with the compounds or combinations of the compounds, the intracellular levels of SAM and SAM metabolites were determined using high performance liquid chromatography. The actual level of the intracellular SAM pool varied considerably (notably after 15 h of incubation), depending on the compound(s) in the incubation solution, but was always highest during inhibition of encystment. SAM was undetectable or barely detectable during stimulation of encystment or under conditions favoring encystment. In cells exposed to L-ethionine, we detected intracellular S-adenosylethionine rather than SAM. The intracellular methionine pool in cells encysting normally for 15 h was found to be 1.3 mumol/pg DNA. Both SAM and S-adenosylhomocysteine (SAH) were present in growing amoebae at concentrations greater than that of cells undergoing normal encystment. The developmental transition from growing amoebae to dormant cysts in P. flavicomum is metabolically characterized by the adjustment of the intracellular concentration of SAM and SAH to a minimal critical level of 0.31 and 0.70 fmol/pg DNA, respectively.