Plasma methionine levels in normal adult subjects after oral loading with L-methionine and N-acetyl-L-methionine.

The biological quality of soy protein isolates is limited by methionine content but can be enhanced by methionine addition. Since supplemental methionine may undergo chemical modification during processing, producing objectionable odors, N-acetyl-L-methionine has been proposed as a methionine replacement. Plasma and erythrocyte methionine levels and the area under the plasma and erythrocyte methionine absorption curve were compared in five normal adult subjects administered equimolar quantities (0.0605 mmoles/kg) of L-methionine and N-acetyl-L-methionine. The two compounds produced an equivalent overall release of L-methionine to the blood as judged by the area under the plasma and erythrocyte methionine time-absorption curves. There was a difference in the early part of both absorption curves (15 to 45 minutes), with plasma and erythrocyte levels higher after L-methionine administration than after N-acetyl-L-methionine administration. However, this difference was only statistically significant at 15 minutes (P = 0.03). Gastric emptying did not account for this difference apparently reflecting a slower rate of absorption of N-acetyl-L-methionine by mucosal cells. No evidence was obtained that indicated release of N-acetyl-L-methionine to the plasma or its excretion in urine after loading. The data are consistent with previous data showing L-methionine and N-acetyl-L-methionine to be equivalent sources of methionine.