N-Methylation of quinolizidine alkaloids: an S-adenosyl-L-methionine: cytisine N-methyltransferase from Laburnum anagyroides plants and cell cultures of L. alpinum and Cytisus canariensis.

An S-adenosyl-L-methionine (SAM): cytisine N-methyltransferase could be demonstrated in crude enzyme preparations from Laburnum anagyroides plants and cell cultures of L. alpinum and Cytisus canariensis. The transferase specifically catalyzes the transfer of a methyl group from SAM to cytisine. The apparent Km values are 60 μmol l(-1) for cytisine and 17 μmol l(-1) for SAM. Other quinolizidine alkaloids, e.g. angustifoline and albine, are N-methylated by only 10-15%. The transferase shows a pH optimum at pH 8.5. It is activated by dithioerythritol and inhibited by thiol reagents and Fe(2+) and Fe(3+). The reaction product S-adenosylhomocysteine is a powerful inhibitor of the transferase reaction. Cell cultures of L. alpinum which have an active SAM: cytisine N-methyltransferase and which are able to N-methylate exogenous cytisine in vivo, do not accumulate cytisine or N-methylcytisine to a detectable degree.