1. Conflicting reports exist as to the organ distribution of betaine-homocysteine methyltransferase (EC It is important to establish its presence or absence in brain, since its substrate, betaine, has recently become established in the treatment of certain diseases involving this organ.

2. It remains unclear whether the reported success of this treatment results from the use of betaine to methylate homocysteine and produce methionine in situ in neural tissue, or whether the effect is secondary to these same reductions happening in other organs, such as the liver. The former would require the presence of betaine-homocysteine methyltransferase in neural tissue.

3. This study demonstrates the complete absence of any activity for this enzyme in the brain of the three species examined. The enzyme was found to be present in both the liver and kidney of man and pig, but only in the liver of the rat.

4. The only source of betaine in cells is via the oxidation of choline. Since the enzymes involved in this conversion have never been shown to exist anywhere other than the mitochondria, it has been assumed that the methyltransferase is also mitochondrial. In this study, it is demonstrated that the enzyme exists only in the cytoplasm of rat liver cells.

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