Long-term Adaptation of Weanling Rats to High Dietary Levels of Methionine and Serine

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Abstract
Long-term adaptation to high levels of methionine was studied in rats. Weanlings were fed an 18% casein diet containing a 2% excess of methionine and/or serine, for periods up to 8 weeks. Methionine was found to be rapidly cleared from the plasma. Excess methionine lowered plasma and liver free threonine levels. Additional serine improved growth and lowered plasma methionine but did not restore threonine levels. Serine, given alone was also found to depress threonine. Taurine levels were elevated by excess methionine. Additional serine initially elevated them further. After a longer period (8 weeks), although serine still lowered methionine, it did not increase taurine any more, nor any of the specific end products of methionine. Hepatic arginase and tyrosine transaminase were stimulated during the first week on high methionine diet, concomitantly with a rise in plasma corticosteroid levels. Serine was efficient in maintaining these activities within control range. By contrast, serine-threonine dehydrase remained stimulated for a longer period and was maintained still longer when serine was added to the diet. The combination of methionine and serine resulted in a more sustained elevation of the enzyme than serine alone. It was concluded that methionine stimulates serine-threonine dehydrase, resulting in a secondary threonine deficiency and in a depression of hepatic serine levels which brings the enzyme back to normal activity. If excess serine is provided the stimulation is maintained, enhancing the threonine deficiency.