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Production of Large Amounts of [¹³C]Leucine-Enriched Milk Proteins by Lactating Cows^1,2,

Yves Boirie, Jacques Fauquant^*, Henri Rulquin, Jean-Louis Maubois^* and Bernard Beaufrère³

Laboratoire de Nutrition Humaine, Université Clermont Auvergne, CRNH, 63009 Clermont-Ferrand Cedex 1, France; ^* Laboratoire de Technologie Laitière, INRA, 35042 Rennes Cedex, France; and Station de Recherches sur la Vache Laitière, INRA, St. Gilles, 35590 L'Hermitage, France

During protein metabolism kinetic studies, oral tracers are administered as labeled free amino acids and do not necessarily represent the metabolic fate of amino acids ingested as proteins. However, sufficient quantities of ¹³C-labeled proteins are not currently available. We here present a new methodology for producing large amounts of milk proteins intrinsically labeled with [¹³C]leucine. After surgical preparation, two lactating cows were infused with 80–90 g of L-[1-¹³C]leucine for 24–32 h, and milk was collected during and after the infusion. Casein and whey protein fractions were purified by membrane separation techniques. Arteriovenous balance across the udder indicated a very efficient extraction of leucine by the mammary gland. Five batches of pure casein and whey proteins, totaling 3854 g of protein of excellent bacteriological quality, were obtained. Two thirds of these proteins had [¹³C]leucine enrichments ranging from 10.5 to 19.4% ([¹³C]leucine atom percent excess). The overall tracer recoveries were 22 and 27% (cows 1 and 2, respectively). Thus, pure milk proteins were produced in large amounts with sufficient ¹³C enrichment to be used in human protein metabolism studies.

KEY WORDS: • leucine • cows • milk • stable isotopes • protein metabolism

¹ Financial support was provided by Ministère de l'Enseignement Supérieur et de la Recherche (grant 91G-0856), Clintec Technologies, Velizy, France and NASA Department (INRA).

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 9 March 1994. Revision accepted 8 June 1994.

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