Identification of peptides in the terminal ileum of broiler chickens fed diets based on maize and soybean meal using proteomics

Abstract

A total of 160 Ross PM3 birds were used in a two treatment feeding study in order to explore the usefulness of proteomics to identify the origin of peptides in ileal digesta. Two diets were fed, one conventional maize/soy-based diet acted as a reference whereas a second diet, formulated to be nutritionally equivalent to the reference diet in protein and energy provision, contained 20% raw soy meal in order to (putatively) elicit changes in intestinal protein flow. Each diet was fed to 10 replicate cages of eight birds per cage from Day 1 to 21. Feed and water were available ad libitum and an indigestible marker was included for assessment of ileal digestibility. Weight gain and feed intake were monitored and at the end of the trial period birds were killed, pancreatic mass was measured and the ileum was excised and the contents were collected, immediately frozen in liquid nitrogen and were subsequently lyophilised. Protein from the ileal digesta was extracted and exposed to proteomic analysis with peptide fragments identified and compared with an amalgamated database containing protein sequences from chicken, soy and maize. Addition of 20% raw soy meal to the maize/soy-based diet resulted in a reduction in weight gain, feed intake and an increase in feed conversion ratio (P < 0.001). Pancreatic mass was significantly increased and the apparent ileal digestibility of protein was significantly decreased by raw soy meal inclusion. Overall, a total of 248 proteins were identified from endogenous origin, 336 from soy and 411 from maize. However, the relative abundance of these proteins were ~20–30% for endogenous protein, 65–75% for soy protein and ~2–4% for maize protein. The addition of 20% raw soy meal resulted in an increase in the relative abundance of endogenous protein and a reduction in the relative abundance of protein from soy with no measurable effect on the presence of protein from maize. Specifically, in the endogenous protein fraction, there was a significant reduction in the relative abundance of metalloendopeptidase, aminopeptidase and alkaline phosphatase and a significant increase in the relative abundance of colipase and trypsin, in response to raw soy meal inclusion. For proteins originating from soybean, the addition of raw soy meal to the diet resulted in a significant increase in the relative abundance of protein from the 2S albumin fraction, Kunitz and Bowman–Birk trypsin inhibitors and soybean agglutinin whereas there was a reduction in the relative abundance of globulin and glycinin. Addition of raw soy meal to the diet also resulted in a significant increase in the presence of maize prolamin in the lumen and a significant decrease in the presence of globulin-2, β-1–3-glucanase and cystatin. These results demonstrate considerable potential of proteomics technology to identify changes in the digestion and secretion of protein in the intestine of chickens. Although these data are preliminary and based on an animal model that included diets that were formulated to have a chronic effect on intestinal physiology it is evident that changes in diet composition can have a profound effect on the origin of protein that leaves the ileum.