Optimisation of enzyme treatment for the degradation of feed proteins for an Escherichia coli auxotroph lysine availability assay

The lysine availability of animal feed sources is dependent upon the protei n source and the processing treatment. Animal bioassays are the standard me thod for evaluating lysine availability. There is a need to improve in vitr o assays for lysine availability to better reflect animal availability meas ures. Such assays could eventually replace the need for animal assays and s ubstantially reduce assay time and costs. One in vitro assay for lysine ava ilability that has been used previously measures the growth response of an Escherichia coli lysine auxotroph on a protein source. However, since E col i does not secrete proteolytic enzymes, it is necessary to predigest protei ns to release free lysine or lysine peptides. The objective of this study w as to determine the lysine availability of different protein sources using different enzyme pretreatments and varying the duration of digestion. Pepsi n, pancreatin, protease and peptidase enzyme solutions were prepared as eit her single enzymes or enzyme combinations and subsequently added to various protein solutions. The lysine availability of the protein digests was assa yed by measuring the growth response of the E coli lysine auxotroph in supp lemented minimal medium. Lysine availability was significantly affected by the enzyme treatment (p < 0.01). The protease-peptidase enzyme combination had the most extensive digestion for all protein sources. There was no sign ificant difference between 4 and 10 h digestion periods for the combination of protease-peptidase. The 4 h protease-peptidase treatment produced a dig est with E coli-determined lysine availability highly correlated (Pearson c orrelation coefficient of 0.942) with animal availability values from previ ous work by others. Based on these results, it is possible to use a standar d 4 h protease-peptidase enzyme treatment to predigest feed proteins to pro vide an accurate estimate of available lysine by an in vitro E coil lysine assay. (C) 1999 Society of Chemical Industry.