Purification of S-adenosylmethionine: epsilon-N-L-lysine methyltransferase. The first enzyme in carnitine biosynthesis.

The initial steps of carnitine biosynthesis in Neurospora crassa involve the methylation of the epsilon-amino group of lysine as follows: Lysine A leads to monomethyllysine B leads to dimethyllysine C leads to trimethyllysine. The methyl donor is S-adenosylmethionine. An enzyme, S-adenosylmethionine:epsilon-N-L-lysine methyltransferase, has been purified from N. crassa to near homogeneity as judged by column chromatography, polyacrylamide gel electrophoresis, and ultracentrifugation. This protein catalyzes all three methylation reactions. The reaction rates are: A less than B less than C. Sedimentation equilibrium and molecular filtration give a molecular weight of 22,000 for the protein. Sedimentation equilibrium analysis of the protein in 6 M guanidine hydrochloride and sodium dodecyl sulfate-polyacrylamide gel electrophoresis do not detect the possibility of subunit structure. The enzyme contains no half-cystine but does contain several acidic residues. The protein exhibits an absorption band between 400 and 420 nm which is 40 to 50 times less than the absorption seen at 280 nm and is not affected by the presence of substrates. The source of this absorption in unkown.