Solubilization and partial purification of S-adenosyl-L-homocysteine binding proteins from the rat brain

Rat brain proteins presenting high-affinity binding of S-adenosyl-L-homocysteine were solubilized and purified. Extraction of binding protein was carried out in the presence of Triton X-100 and 1 M NaCl; this solubilized fraction exhibits similar kinetic properties than the membrane proteins. Purification was performed using affinity chromatography on S-adenosyl-L-homocysteine carboxyhexyl Sepharose 48 conjugate. The analysis of the affinity gel eluate by SDS-PAGE showed high purification ratios for two proteins exhibiting 54 and 68 kDa. Three activity peaks were separated when solubilized membrane proteins were submitted to isoelectric focusing; the activity peaks corresponded to proteins of pH1 6.0, 6.5, and 7.2. SDS-PAGE separation of proteins contained in each peak showed protein aggregation; a 54-kDa subunit was present in each aggregate. Solubilized membrane proteins were labeled by photoaffinity labeling with tritiated S-adenosyl-L-homocysteine; the 54- and 68-kDa proteins were found among the specifically labeled proteins. Finally, according to the previous data from the literature, the purified S-adenosyl-L-homocysteine binding proteins do not seem to be the same as adenosine receptors or phosphatidylethanolamine-N-methyltransferase.