Radioactive Phosphorus as an Indicator of Phospholipid Metabolism Xii. Further Observations on the Effects of Amino Acids on Phospholipid

Three amino acids have been shown to accelerate phospholipid turnover in the liver: methionine, cystine, and cysteine (1). In short intervals after the administration of radioactive phosphorus, greater amounts of labeled phospholipid are deposited in the livers of rats fed these three amino acids than in the livers of control animals. In the case of methionine, this finding lends support to the view that the rate of phospholipid turnover is a factor determining the fat content of the liver. Methionine stimulates phospholipid turnover (1) and prevents the deposition of excessive amounts of fat in the liver (2, 3). The observation, however, that cystine and cysteine stimulate phospholipid turnover appears incompatible with their lipotropic actions, for prolonged feeding of both does not prevent fatty infiltration in the liver. Although this discrepancy merits further investigation, it was nevertheless pointed out that an increased phospholipid turnover was observed in this laboratory after single feedings of cystine or cysteine, whereas lipotropic actions were detected only after their prolonged administration (4). Deleterious effects have repeatedly been observed after prolonged feeding of cystine (5-7). In this connection it would be of interest to know whether a single feeding of cystine could produce a lipotropic effect, but it seems very unlikely at present that a change in the fat content of the liver could be detected after so short an interval.