D-Methionine Utilization During Parenteral Nutrition in Adult Rats

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Abstract
D-Methionine, present in some parenteral solutions, is poorly utilized by man. To evaluate some of the factors affecting utilization, methionine isomer excretion was measured in rats treated with two different parenteral regimens: total parenteral nutrition and protein sparing therapy. Within each regimen, nutrient solutions differing only in methionine isomer composition were compared. Eight rats on the total parenteral nutrition regimen were infused with a 33% glucose—2.8% amino acid solution at a rate providing 100% of energy requirement. Four rats received the parenteral solution containing L-methionine, and four the solution containing DL-methionine. Eight additional rats on a protein sparing regimen were infused with an amino acid solution (8.5%) at a rate to provide 25% of energy requirement. Four rats were infused with a solution containing L-methionine, and four with a solution containing DL-methionine. Total methionine content of livers was not affected by differences in methionine isomer. Infusion of the solution containing DL-methionine significantly increased plasma methionine levels during protein sparing therapy, with the increase representing accumulation of the D-isomer. Rats receiving protein sparing therapy had significantly higher liver methionine levels than rats receiving total parenteral nutrition. Although rats infused with the solution containing DL-methionine had elevated plasma methionine levels during protein sparing therapy, urinary methionine losses were small, as were losses of methionine sulfoxide and α-keto-γ-methiolbutyrate. In contrast to humans, adult rats utilized more than 99% of parenterally administered D- or L-methionine.