Abstract
The isoelectric point (p__I__) of S‐adenosyl‐L‐methionine and some of its metabolic products has been determined by an isoelectric focusing technique. The experimental p__I__ values of S‐adenosyl‐L‐methionine and S‐methylmethionine are 7.24 and 6.85, respectively, and the p__I__ values of S‐adenosyl‐L‐homocysteine and homocysteine are 6.05 and 5.65, respectively. This indicates that the contribution by adenosine, adenine and ribose to the total electric charge of S‐adenosyl‐L‐methionine or S‐adenosyl‐L‐homocysteine is approximately 0.40 p__I__ unit. The differences in the p__I__ values between two pairs of compounds (S‐adenosyl‐L‐methionine vs. S‐adenosyl‐L‐homocysteine and S‐methylmethionine vs. methionine) are very close (1.19 and 1.14, respectively). This difference is the amount of p__I__ value contributed by the sulfonium group which has been formed by methyl addition. Surprisingly, the presence of a permanent positive charge at the sulfonium center in S‐adenosyl‐L‐methionine and S‐methylmethionine does not result in a higher p__I__ values for these compounds. This might be due to the partial neutralization of the positive charge by the formation of carbanion. The mechanism of charge neutralization in these sulfonium compounds is discussed.