Morphology, proliferation, and ribonucleic acid and fractional protein syntheses in the small intestinal mucosa of young goats fed soy protein-based diets with or without amino acid supplementation

The study was designed to examine whether feeding soy protein isolate as partial replacement of casein (CN) affects jejunal protein synthesis and whether effects may be ameliorated by supplementation of those AA known to be at lower concentrations in soy protein isolate than in CN. Goat kids (14 d) were fed comparable milk protein diets, in which 50% of the crude protein was CN (CAS), soy protein isolate (SPI), or soy protein isolate supplemented with AA (SPIA) for 43 d (n = 8 per group). On d 42, plasma concentrations of protein, urea, and AA were measured before and after morning feeding. In the morning of d 43, [15N]RNA from yeast [13 mg/kg of body weight (BW)] was given with the diet to measure the reutilization of dietary RNA precursors for mucosal RNA biosynthesis. Four hours later, an oral dose of l-[1-13C]leucine (180 mg/kg of BW) was administered and blood samples were collected between −15 and +45 min relative to tracer administration for analysis of plasma 13C α-ketoisocaproic acid and 13C recovery in blood CO2. Kids were killed 60 min after the tracer application, and jejunal tissue was collected to determine mucosal morphology, cell proliferation, enzyme activities, RNA synthesis, and fractional protein synthesis rate. Plasma protein concentrations were higher in CAS than in SPI and SPIA. Plasma concentrations of Thr were higher in CAS than in SPI and SPIA, and those of Met were lower in SPI than in CAS and SPIA. In mid-jejunum, villus circumferences were higher in CAS than in SPI and SPIA, and villus height and villus height:crypt depth ratio were higher in CAS than in SPI. In mid-jejunum, mucosal protein concentrations were higher in CAS than in SPI and SPIA and mucosal activities of aminopeptidase N tended to be higher in CAS than in SPI, whereas activities of dipeptidyl peptidase IV tended to be lower in SPI than in SPIA. Activities of 5′ nucleotidase and xanthine oxidase were lower in CAS than in SPI. The 13C recovery in blood CO2 tended to be higher in SPI than in CAS. In mid-jejunum, 15N enrichment of RNA tended to be higher in CAS than in SPI, and 13C enrichment of protein-bound Leu was higher in SPI than in CAS. In mid-jejunum, the fractional protein synthesis rate tended to be higher in SPI than in CAS. Our results revealed changes in intestinal growth after soy protein feeding that were associated with effects on intestinal RNA and protein synthesis but that were not ameliorated by AA supplementation.