Effect of inhibitors of transmethylation on histamine release from human basophils

Methylation reactions mediated by $\text{S-}\text{adenosyl-}\text{l}\text{-methionine}$ (AdoMet) play an important role in a number of biological reactions including bacterial and human monocyte chemotaxis. This study evaluated the role of methylation reactions in histamine release from human basophils. Methylation was blocked by several methods. Erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), an inhibitor of adenosine deaminase (EC 3.5.4.4), blocked the IgE-mediated histamine release with an ic₅₀ (concentration of drug required to produce 50 per cent inhibition) of 0.33 mM. Preincubation of leukocytes with adenosine caused some inhibition of IgE-mediated histamine release. This inhibition by adenosine was potentiated by the addition of 1 × 10^?5 M EHNA which alone did not affect histamine release. Further addition of l-homocysteine thiolactone at 1 × 10^?4 M potentiated the inhibitory effect of EHNA plus adenosine. The effect of l-homocysteine thiolactone was dose dependent. 3-Deazaadenosine (DZA), an inhibitor of $\text{S-}\text{adenosyl-}\text{l}\text{-homocysteine}$ hydrolase (EC 3.3.1.1) inhibited IgE-initiated histamine release from human basophils with an ic₅₀ of ~1 mM. This inhibition was potentiated by l-homocycsteine thiolactone. The inhibition by DZA was not potentiated by two different phospho-diesterase inhibitors suggesting that the action of DZA was not through changes in intracellular levels of cyclic AMP. DZA inhibited during the Ca²⁺-independent activation step of IgE-mediated basophil histamine release. In contrast, when histamine release was induced by the calcium ionophore A23187, f'Met-Leu-Phe or zymosan-activated serum, there was either no inhibition or enhancement of histamine release by these inhibitors of transmethylation. These results suggest that AdoMet-mediated methylation plays an important role in IgE-initiated histamine secretion from human basophils and that release induced by A23187, fMet-Leu-Phe, or C5A, bypasses this reaction step.