Branched-chain amino acids (AA) are mostly metabolized in the splanchnic area, but some are metabolized within the mammary gland and thus could contribute to the synthesis of non-essential AA (Wohlt et al. 1977). The extraction rate of leucine from plasma by the mammary gland is particularly high (64% in the goat; Roets et al. 1983), in excess of that used for the synthesis of milk proteins (Davis & Mepham, 1976; Wohlt et al. 1977; Roets et al. 1983). Thus, although mammary leucine is mainly used for milk protein, it also provides amino nitrogen and carbon sources for the synthesis of non-essential AA.

To our knowledge, no information is available on the transfer and distribution of plasma leucine amino nitrogen to milk protein AA. Using the technique for chromatographic fractionation of AA recently developed by Casseron et al. (1997), we studied the specific 15N enrichment of casein (CN) AA in the goat given a single intravenous injection of [15N]leucine.