Methionine Metabolism in Piglets Fed dl-Methionine or Its Hydroxy Analogue Was Affected by Distribution of Enzymes Oxidizing These Sources to Keto-Methionine

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Abstract
Previous evidence shows that the extensive catabolism of dietary essential amino acids (AA) by the intestine results in decreased availability of these AA for protein synthesis in extraintestinal tissues. This raises the possibility that extraintestinal availability of AA may be improved by supplying the animal with an AA source more of which can bypass the intestine. To test this hypothesis, six barrows (35-day-old, 8.6 ± 1.4 kg), implanted with arterial, portal, and mesenteric catheters, were fed a dl-methionine (dl-MET) or dl-2-hydroxy-4-methylthiobutyrate (dl-HMTB) diet once hourly and infused intramesenterically with 1% p-amino hippurate. Although the directly available l-MET in dl-MET diet was about 1.2-fold that in dl-HMTB diet, the net portal appearance of l-MET was not different between the two diets. Compared with the low mRNA abundance and low activity of d-2-hydroxy acid dehydrogenase (d-HADH) and l-2-hydroxy acid oxidase (l-HAOX) in the intestine, the high mRNA abundance and high activity of d-AA oxidase (d-AAOX) indicated that the intestine had a relatively higher capacity of d-MET utilization than of dl-HMTB utilization to l-MET synthesis and its subsequent metabolism. However, in contrast to the much lower d-AAOX activity (nmol/g tissue) in the stomach than in the liver and kidney, both d-HADH and l-HAOX activity in the stomach was comparable with those in the liver and/or kidney, indicating the substantial capacity of the stomach to convert dl-HMTB to l-MET. Collectively, the difference in distribution of activity and mRNA abundance of d-AAOX, d-HADH, and l-HAOX in the piglets may offer a biological basis for the similar portal appearance of l-MET between dl-MET and dl-HMTB diets, and thus may provide new important insights into nutritional efficiency of different l-MET sources.

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