Characteristics of protein carboxyl methylation in the rat hypothalamus

The formation of methyl-labeled S-adenosylmethionine (AdoMet) and methyl esters of endogenous methyl-acceptor proteins (MAPs) was studied in a synaptosomal preparation from the rat hypothalamus labeled with L-[methyl-3H]methionine. Incubation of synaptosomes with L-[methyl-3H]methionine resulted in a rapid labeling of the AdoMet pool and a less rapid formation of 3H-methyl-MAPs. Accumulation of 3H-methyl-MAPs was linear over a 30-min period. The effects of various inhibitors of Ado-Met-dependent transmethylation reactions on the formation of carboxylmethylated MAPs were examined. When hypothalamic synaptosomes were preincubated with L-[methyl-3H]methionine and subsequently incubated for 30 min in the presence of S-adenosyl-L-homocysteine (AdoHcy, 100 microM), 3H-methyl-MAP formation was inhibited by approximately 70%, 100 microM-L-homocysteine thiolactone (HTL) as well as 100 microM-3-deazaadenosine (c3Ado) also caused a 60--70% inhibition of 3H-methyl-MAP formation; the combination of both c3Ado and HTL produced a slightly but not significantly greater inhibition than either agent alone. 10 microM-adenosine or 10 microM-HTL each produced an approximately 40% inhibition of 3H-methyl-MAP formation: the inhibitory effect of the two agents in combination was additive. Sinefungin and A9145C, potent inhibitors of bovine adrenomedullary protein carboxyl methylase, had no effect on 3H-methyl-MAP formation in hypothalamic synaptosomes at concentrations up to 1 mM. However, these compounds were potent inhibitors of 3H-methyl-MAP formation in lysed synaptosomes incubated with [3H-methyl]AdoMet. These results demonstrate that hypothalamic synaptosomes are capable of methionine activation and protein carboxyl methylation.