Decreased Transmethylation of Biogenic Amines After In Vivo Elevation of Brain S‐Adenosyl‐l‐Homocysteine

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Abstract
The ability of S‐adenosyl‐l‐homocysteine (AdoHcy) to inhibit biologic transmethylation reactions in vitro has led us to explore the possibility of pharmacologically manipulating AdoHcy levels in vivo and examining the consequences of these alterations on the transmethylation of some biogenic amines. Swiss‐Webster mice were injected intraperitoneally with different doses of adenosine (Ado) and d,l‐homocysteine thiolactone (Hcy) and were killed at various times thereafter. S‐Adenosyl‐l‐methionine (AdoMet) and AdoHcy concentrations were determined by using a modified isotope dilution‐ion exchange chromatography‐high pressure liquid chromatography technique sensitive to less than 10 pmol. Increasing doses of Ado + Hcy (50‐1000 mg/kg of each) produced a dose‐related increase in blood, liver, and brain AdoHcy levels. At a dose level of 200 mg/kg Ado + Hcy, AdoHcy levels were markedly elevated, with minimal concomitant perturbations of AdoMet. This elevation was maximal 40 min after giving Ado + Hcy, returning to control values within 6 h. Ado + Hcy treatment resulted in decreased activities of catechol‐O‐methyltransferase, histamine‐N‐methyltransferase, and AdoHcy hydrolase in vitro. The cerebral catabolism of intraventricularly administered [3H]histamine (HA) was decreased in a dose‐related manner by Ado + Hcy treatment as evidenced by higher amounts of nonutilized [3H]HA in brain, concurrent decreases in [3H]methylhistamine formation, and decreases in the transmethylation conversion index. Steady state levels of HA also showed dose‐related increases after Ado + Hcy treatment. It is concluded that injections of Ado + Hcy can markedly elevate AdoHcy levels in vivo, which can, in turn, decrease the rate of transmethylation reactions.

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