A site-directed-mutagenesis study of putative active-site residues in rat liver betaine—homocysteine S-methyltransferase has been carried out. Identification of these amino acids was based on data derived from a structural model of the enzyme. No alterations in the CD spectra or the gel-filtration chromatography elution pattern were observed with the mutants, thus suggesting no modification in the secondary structure content or in the association state of the proteins. All the mutants obtained showed a reduction of the enzyme activity, the most dramatic effect being that of Glu159, followed by Tyr77 and Asp26. Changes in affinity for either of the substrates, homocysteine or betaine, were detected when substitutions were performed of Glu21, Asp26, Phe74 and Cys186. Interestingly, Asp26, postulated to be involved in homocysteine binding, has a strong effect on affinity for betaine. The relevance of these results is discussed in the light of very recent structural data obtained for the human enzyme.

Abbreviations used: ADA, adenosine deaminase; AdoHcy, S-adenosylhomocysteine; AdoMet, S-adenosylmethionine; BHMT, betaine—homocysteine S-methyltransferase; CB-Hcy, S-(Δ-carboxybutyl)-l-homocysteine; CBS, cystathionine β-synthase; E21K, Glu21→Lys mutation; Hcy, homocysteine; SS, secondary structure.

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