Comparative metabolism of L-methionine and N-acetylated derivatives of methionine.

These experiments compared the metabolism of the N-acetylated derivatives of D- or L-methionine to that of L-methionine. Spargue-Dawley rats were orally or intraperitoneally dosed with N-[1-14C]acetyl-L-methionine, N-[1-14C]acetyl-D-methionine, or sodium [1-14C]acetate. 14CO2 was collected at intervals over 24 hours. In addition, groups of rats were orally dosed with either 35S-labeled N-acetyl-L-methionine or L-methionine. The animals were killed 3, 24, and 168 hours after dosing. Urine and feces were collected, and tissues were excised for 35S determinations. With either route of dosing, N-[1-14C]acetyl-L-methionine yielded the same amount of 14CO2 as sodium [1-14C]acetate over a 24-hour period. The acetate moiety of N-[1-14C]acetyl-D-methionine is not readily metabolized to 14CO2. Within each time period after dosing, the tissue distribution of 35S from 35S-labeled N-acetyl-L-methionine and L-methionine was similar. Protein specific activities for the two isotopes were also the same. After 168 hours, 30% of both isotopes of 35S appeared in the urine and feces, and the two isotopes were similarly distributed in the organic -S and inorganic -S fractions of urine. The studies show that L-methionine from N-acetyl-L-methionine is metabolically equivalent to free L-methionine. This conclusion is consistent with rat feeding studies showing that N-acetyl-L-methionine is nutritionally equivalent to L-methionine.