The mechanism of histamine methyltransferase (HMT) inhibition by S‐adenosylhomocysteine (SAH) has been investigated on a partially purified enzyme from guinea‐pig brain. The kinetic data indicated that this inhibition was competitive with respect to S‐adenosylmethionine (SAMe) and noncompetitive with respect to histamine. The Kt, values (about 5 ± 10−6 M in both cases) indicated that SAH had a higher affinity than SAHe or histamine for the enzyme. In rats, after administration of a small dose of SAH, methylation of intracisternally injected [3H]histamine was not modified. However, treatment with l‐DOPA or pyrogallol induced a decrease in [3H]histamine methylation, presumably due to a modification in the SAMe/SAH ratio in the brain. Hence, histamine methylation in brain could be regulated according to the value of this ratio and thus related to methylation of other biogenic amines.
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Baudry et al. (1973) studied this question.
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