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Homoarginine Labeling is Suitable for Determination of Protein Absorption in Miniature Pigs^{1, 2,}

Institut für Physiologie und Biochemie der Ernährung, Bundesanstalt für Milchforschung, D-2300 Kiel, Germany ^* Institut für Humanernährung und Lebensmittelkunde, Universität Kiel, Germany

Intestinal proteolysis and absorption of dietary protein was followed using a chemical label of the lysine side-chain. Protein-bound lysine was transformed into homoarginine (HA), an amino acid that is not used for protein synthesis. Disappearance in the intestinal tract of HA originating from labeled casein and soybean isolate and appearance in peripheral blood was followed in four miniature pigs fitted with permanent T-cannulas at the proximal jejunum and ileum. Less than 0.2% of HA appeared in the digesta at the ileum when up to 13.5 mmol of HA was infused either into the jejunum or jugular vein. Several factors suggest that HA in fact traces the exogenous (dietary) protein: 1) the quick postprandial appearance of HA in plasma; 2) the high (>97%) oro-ileal absorption of HA-labeled casein and soybean isolate; 3) the similarity of chymotryptic proteolysis in vitro of guanidinated and native casein; 4) the fact that only trace amounts of the marker reenter the intestinal lumen from the blood. Therefore HA-label is suitable for the differentiation of exogenous and endogenous protein in the chyme and thus the measurement of oro-ileal protein absorption and irreversible loss of endogenous protein in the chyme of the small intestine.

KEY WORDS: • homoarginine • protein absorption • protein digestibility • endogenous protein • miniature pigs

¹ Part of this work was presented at a meeting of the Nutrition Society in London [Hagemeister, H. & Erbersdobler, H. (1985) Chemical labeling of dietary protein by transformation of lysine to homoarginine: a new technique to follow intestinal digestion and absorption. Proc. Nutr. Soc. 44: 133A] and at the 2nd Symposium on "Milk Proteins" in Kiel, Germany [Hagemeister, H., Schmitz, M. & Erbersdobler, H. (1988) Reliability and limitations of the homoarginine method for evaluation of protein digestibility in the pig. In: Milk Proteins (Barth, C. A. and Schlimme, E., eds.), pp. 68–71, Springer-Verlag, New York, NY].

² Supported by the Deutsche Forschungsgemeinschaft (Grant HA 456/2.1 II B 8).

³ Present address: IS Forschungsgesellschaft für experimentelle Tierphysiologie und Tierernährung, P.O.B. 1247, 2362 Wahlstedt, Germany.

⁴ To whom correspondence should be addressed.

Manuscript received 6 November 1990. Revision accepted 26 March 1991.

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