Evaluation of a representation of the limiting amino acid theory for milk protein synthesis.

Abstract

AbstractA model describing amino acid (AA) metabolism by the mammary glands of the lactating cow has been constructed (Hanigan et al., 2000a). Milk protein production was predicted using a mathematical procedure to determine which among histidine (His), lysine (Lys), methionine (Met), threonine (Thr) and tyrosine plus phenylalanine (TP) was most limiting for milk protein synthesis. The minimum protein synthetic flux determined the overall rate of protein synthesis. The ability of the model to predict substrate removal and milk protein output was assessed, using the parameterization data (reference data) and an independent data set assembled from the literature (literature data). When the reference data were simulated, the model generally fitted the uptake data well. However, the model predicted milk protein yields poorly. Of the four experiments contained in the reference data set, only one experiment (C6) contained complete data for all driving AA. The model accounted for 53% of the observed variation in milk protein yields for C6, suggesting that inadequate data were the cause of inaccurate simulations for the remaining experiments in the reference data set. The model explained 43% of the observed variation in milk protein yields when the literature data set was simulated. Adoption of an alternative representation of milk protein synthesis, wherein all five driving AA affected milk protein synthesis simultaneously in a linear additive manner, resulted in a reduction in the accuracy of predictions of milk protein yields when C6 or the literature data set was simulated. Use of a Michaelis-Menten equation form to describe milk protein synthesis resulted in slight improvements in accuracy when the C6 data set was simulated and a reduction in accuracy when the literature data set was simulated. After fitting sensitivity coefficients for a modified Michaelis-Menten equation to the literature data, the model described 60% of the observed variation in milk protein output. Attempts to derive sensitivity coefficients for the linear additive equation were unsuccessful, due to model instability caused by the equation. Based on the results herein, a modified version of the Michaelis-Menten equation appeared to represent the effects of essential AA effects on milk protein synthesis better than an equation considering a single limiting AA.