Quantitative evaluation and regulation of S-adenosylmethionine-dependent transmethylation in sheep tissues

The overall rates of S-adenosylmethionine (AdoMet)-dependent transmethylation were estimated in various tissues from the initial rate of S-adenosylhomocysteine (AdoHcy) plus AdoMet accumulation after blocking hydrolysis of AdoHcy. The rates were found to differ widely among the tissues of sheep and the highest rate was in the pancreas, being 600 times higher than that in the muscle. Sheep liver possessed approximately 75% of total-body capacity for transmethylation although the transmethylation rate was approximately half that in rat liver. The minimum estimate of daily requirement of AdoMet for transmethylation for adult sheep was approximately 18 mmol, far in excess of methionine intake. Methionine loading elevated AdoMet levels only in the tissues with a high or moderate rate of transmethylation. The kinetic properties of major methyltransferases in sheep liver along with tissue distribution of AdoMet and AdoHcy suggest that transmethylation rate is subject to physiological regulation by tissue levels of AdoMet and AdoHcy.