1. Methionine synthetase was measured in the mucosal cells of the rat duodenum, jejunum and ileum by a previously employed method for mucosal cell isolation. No activity was found in these cells.

2. When a dual buffer system for the isolation of villous and crypt cell population was substituted, however, methionine synthetase was found to be active in the duodenum, jejunum and ileum, both in the villous and crypt cell populations. The activity was significantly higher in the crypt cells than in the villous cells throughout the intestine, and higher levels were found in the ileum than in the duodenum or jejunum.

3. As had been previously reported for the rat liver, nitrous oxide in vivo reduced the enzyme activity in both the villous and crypt cell populations, suggesting a role in vivo for the enzyme. We conclude that methionine synthetase is both present and active in the small intestinal mucosal cells of the rat.

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