Modulation by the ratio S-adenosylmethionineS-adenosylhomocysteine of cyclic AMP-dependent phosphorylation of the 50 kDa protein of rat liver phospholipid methyltransferase

The present results show that the catalytic subunit of cyclic AMP-dependent protein kinase phosphorylates the 50 kDa protein of rat liver phospholipid methyltransferase at one single site on a serine residue. Phosphorylation of this site is stimulated 2- to 3-fold by S-adenosylmethionine. S-adenosylmethionine-dependent protein phosphorylation is time- and dose-dependent and occurs at physiological concentrations. S-adenosylhomocysteine has no effect on protein phosphorylation but inhibits S-adenosylmethionine-dependent protein phosphorylation. $\text{S-}\text{Adenosylmethionine}\text{S-}\text{adenosylhomocysteine}$ ratios varying from 0 to 5 produce a dose-dependent stimulation of the phosphorylation of the 50 kDa protein. In conclusion, these results show, for the first time, that the ratio $\text{S-}\text{adenosylmethionine}\text{S-}\text{adenosylhomocysteine}$ can modulate phosphorylation of a specific protein.