Methionine Infusion Reproduces Liver Injury of Parenteral Nutrition Cholestasis

Cholestatic jaundice is the major complication of total parenteral nutrition (TPN) in infancy. We have previously shown that the TPN solution is directly toxic to the liver, and that this toxicity appears to be mediated by one or more amino acids. Elevated serum methionine levels, without corresponding increases in its metabolites, suggest that accumulation of this toxic amino acid may cause TPN cholestasis. Nine-week-old rabbits (n = 28) were divided into three groups. The FED group was fed standard rabbit chow ad libitum. The TPN group was not fed and received only i.v. TPN (including methionine 121 mg·kg-1·d-1), and lipids. The EXP group was fed chow ad libitum and received i.v. methionine (121 mg·kg-1·d-1). After 14 d, we evaluated bile flow, bromosulfophthalein excretion, serum liver enzymes, liver histology, and serum amino acid levels. Bile flow was significantly depressed in the TPN and EXP groups compared with FED controls (32.9 ± 9.4 and 45.7 ± 14.4 versus 82.9 ± 13.8). Excretion of the bilirubin analog bromosulfophthalein tended to be delayed by methionine infusion (p = 0.15). Serum liver enzymes (aspartate transaminase, alanine aminotransferase, γ-glutamyltransferase, and alkaline phosphatase) were normal in all groups. Histologic liver injury in the EXP group was similar to that caused by TPN. Balloon degeneration, and portal inflammation were seen in both groups. Homocysteine, an early metabolite of methionine, was elevated in the TPN and EXP groups compared with FED controls. Intravenous methionine is hepatotoxic. Despite full oral feeding, it produces a depression of bile flow and histologic liver injury similar to that seen with TPN. Elevated homocysteine levels suggests an enzymatic block early in the pathway of methionine metabolism. We believe that methionine may be an important factor in the pathogenesis of TPN cholestasis.