Purification and properties of rat lens methionine adenosyltransferase

Methionine adenosyltransferase (MAT) has been partially purified from rat lenses using a combination of ammonium sulfate fractionation and hydrophobic chromatography on phenyl Sepharose columns. The partially purified enzyme resembles purified Type II MAT from non-hepatic tissues. The K m for methionine is 3·0 μ m, and the K m for ATP is 80 μ m. The enzyme is activated by potassium ions (25–50 m m), and inhibited by higher concentrations of potassium. A divalent cation (magnesium or manganese) is essential for activity. The V max with magnesium is about five times higher than with manganese, but the optimal manganese concentration is around 2·0 m m, compared with 10–20 m m for magnesium. The enzyme is active over a broad pH range, with optimal activity between pH 7·0 and 8·0. The enzyme is inhibited by all three of its products, phosphate, pyrophosphate, and S-adenosylmethionine. Individually phosphate and pyrophosphate are weak inhibitors, but in combination they show a marked synergistic inhibitory effect. Tripolyphosphate is also an effective inhibitor. The inhibition of the enzyme by the cataractogenic agent, dimethylsulfoxide, further confirmed the similarity to Type II MAT.