Limiting Amino Acids for Milk Protein Synthesis By Bovine Mammary Cells in Culture

Abstract

Experiments are presented to determine the effect of labeled precursor amino acid on synthesis of (β-lactoglobulin and β-casein and to identify the most limiting amino acids in Eagle's minimal essential medium for synthesis of milk protein. Protein synthesis was measured by incorporation of carbon-14 labeled amino acids into milk protein from bovine mammary cell culture. Mammary tissue from 13 Holsteins (12.7kg milk/day) was dispersed and cultured for 18h. In Experiment 1, three replications (cows) with three amino acid mixtures, utilizing labeled lysine or leucine as the marker amino acid, showed that labeled lysine overestimated the effect of medium lysine, and labeled leucine overestimated the effect of leucine on (β-lactoglobulin and (β-casein synthesis. Subsequently Experiment 2 utilized 10 replications (cows) to determine the effect of a three-fold increase of each of 13 amino acids in Eagle's medium. Increased threonine or methionine improved β-lactoglobulin synthesis and cystine increased β-casein. Second and third best response in β-casein were from addition of methionine and threonine. Combined analysis from 10 cows and two protein responses by multivariate procedures identified, in order of decreasing magnitude, cystine, threonine, and methionine as limiting amino acids in the dispersed cell culture.