Protein carboxyl methylation in synaptic membrane of rat brain: the possible presence of adenosine-bound S-adenosyl-L-homocysteine hydrolase in the membrane.

Abstract

The effects of some neurotransmitters, adenosine (Ad), and homocysteine (Hcys) on protein carboxyl methylation in synaptic plasma membranes from rat cerebral cortex were examined. Neither any of the neurotransmitters nor Ad had a detectable effect. Incubation of membrane with DL-Hcys alone (5 X 10(-5) M), the combination of both Ad (5 X 10(-5)) and DL-Hcys (5 X 10(-5)), or S-adenosyl-L-homocysteine (SAH) (1 X 10(-6)) strongly decreased the methyl ester formation. The inhibitory effect of the combination of both compounds may be interpreted in terms of the increased SAH concentration due to the presence of SAH hydrolase in the membrane. The inhibitory effect of Hcys alone was blocked by preincubation with Ad deaminase or Neplanocin A, a potent inhibitor of SAH hydrolase, suggesting the presence of Ad-bound SAH hydrolase in the synaptic membrane. Ad-bound SAH hydrolase activity estimated by the inhibition of methylation in the presence of Hcys was located in the membrane fractions including synaptosomes, myelin, and microsomes (about 70%), but the SAH hydrolase activity estimated on the basis of the inhibitory effect of the combination of both Ad and Hcys was localized exclusively in the soluble fraction (about 90%). The distribution of the latter activity is coincident with that of SAH hydrolase reported to date. Incubation of the synaptic membrane with Hcys markedly increased the SAH concentration. The stimulatory effect of Hcys alone was blocked by Ad deaminase.