Equine enterocytes actively oxidize l-glutamine, but do not synthesize l-citrulline or l-arginine from l-glutamine or l-proline in vitro. (11th March 2022)

• Record Type: Journal Article
• Authors: Martinez, Rafael E; Leatherwood, Jessica L; Bradbery, Amanda N; Silvers, Brittany L; Fridley, Jennifer; Arnold, Carolyn E; Posey, Erin A; He, Wenliang; Bazer, Fuller W; Wu, Guoyao
• Is Part Of: Journal of animal science. Volume 100:Number 4(2022)
• Publication Date: 2022-03-11
• Subjects: arginine -- citrulline -- enterocyte -- equine -- ornithine -- small intestine; Livestock -- Periodicals; Livestock; Electronic journals; Periodicals; 636.005
• DOI: 10.1093/jas/skac077
• Languages: English
• ISSNs: 0021-8812

Abstract

Abstract: In livestock species, the enterocytes of the small intestine are responsible for the synthesis of citrulline and arginine from glutamine and proline. At present, little is known about de novo synthesis of citrulline and arginine in horses. To test the hypothesis that horses of different age groups can utilize glutamine and proline for the de novo synthesis of citrulline and arginine, jejunal enterocytes from 19 horses of three different age groups: neonates ( n = 4; 7.54 ± 2.36 d of age), adults ( n = 9; 6.4 ± 0.35 yr), and aged ( n = 6; 22.9 ± 1.0 yr) with healthy gastrointestinal tracts were used in the present study. Enterocytes were isolated from the jejunum and incubated at 37 °C for 30 min in oxygenated (95% O2 /5% CO2 ) Krebs bicarbonate buffer (pH 7.4) containing 5 mM D-glucose and 0 mM, 2-mM L-[U- 14 C]glutamine, or 2 mM L-[U- 14 C]proline plus 2 mM L-glutamine. Concentrations of arginine, citrulline, and ornithine in cells plus medium were determined using high-performance liquid chromatography. Results indicate that the rate of oxidation of glutamine to CO2 was high in enterocytes from neonatal horses, but low in cells from adult and aged horses. Enterocytes from all age groups of horses did not degrade proline into CO2 . Regardless of age, equine enterocytes formed ornithine from glutamine and proline, but failed to convert ornithine into citrulline and arginine. Because arginine is an essential substrate for the synthesis of not only proteins, but also nitrogenous metabolites (e.g., nitric oxide, polyamines, and creatine), our novel findings have important implications for the nutrition, performance, and health of horses. Lay Summary: The amino acid arginine (Arg) is a precursor for the synthesis of multiple biological molecules including nitric oxide, polyamines, and creatine that are involved in cell proliferation, cellular remodeling, dilation of blood vessels, and phosphocreatine production for a readily available source of energy. Multipurpose capabilities of Arg have increased the interest in its effects in other species and must be evaluated in the horse. Levels of Arg are deficient in the milk of mammals such as humans, cows, sheep, and pigs, but their neonates are capable of synthesizing citrulline and Arg from glutamine and proline in the small intestine. High concentrations of Arg in milk have been observed in the horse, warranting investigation in case that the foal cannot synthesize Arg to support growth and thus rely on milk as the sole source of Arg. To date, no research has determined the endogenous production of Arg in horses to support metabolic and physiological processes; therefore, our experiment quantifies the synthesis of Arg in enterocytes of the small intestine of neonatal, adult, and aged horses. Data collected from this study serve as the necessary first step to determine the Arg requirement in the horse that has over-reaching implications to improve the growth, performance, reproductive efficiency, and to enhance longevity of the horse. Abstract : Establishing a specific dietary arginine requirement for the horse has yet to be investigated; therefore, the requirement for supplemental arginine in the equine diet should be determined by experimentally quantifying the endogenous synthesis of this amino acid to address dietary deficiencies.