Proline and hydroxyproline metabolism: implications for animal and human nutrition

Abstract

Proline plays important roles in protein synthesis and structure, metabolism (particularly the synthesis of arginine, polyamines, and glutamate via pyrroline-5-carboxylate), and nutrition, as well as wound healing, antioxidative reactions, and immune responses. On a per-gram basis, proline plus hydroxyproline are most abundant in collagen and milk proteins, and requirements of proline for whole-body protein synthesis are the greatest among all amino acids. Therefore, physiological needs for proline are particularly high during the life cycle. While most mammals (including humans and pigs) can synthesize proline from arginine and glutamine/glutamate, rates of endogenous synthesis are inadequate for neonates, birds, and fish. Thus, work with young pigs (a widely used animal model for studying infant nutrition) has shown that supplementing 0.0, 0.35, 0.7, 1.05, 1.4, and 2.1% proline to a proline-free chemically defined diet containing 0.48% arginine and 2% glutamate dose dependently improved daily growth rate and feed efficiency while reducing concentrations of urea in plasma. Additionally, maximal growth performance of chickens depended on at least 0.8% proline in the diet. Likewise, dietary supplementation with 0.07, 0.14, and 0.28% hydroxyproline (a metabolite of proline) to a plant protein-based diet enhanced weight gains of salmon. Based on its regulatory roles in cellular biochemistry, proline can be considered as a functional amino acid for mammalian, avian, and aquatic species. Further research is warranted to develop effective strategies of dietary supplementation with proline or hydroxyproline to benefit health, growth, and development of animals and humans.

Fig. 1

Structures of proline and related metabolites. All of these proline metabolites occur in animals, with 4-hydroxyproline being the most abundant

Fig. 2

Metabolism of 4-hydroxyproline in animals. AA amino acid, AGT alanine-glyoxylate aminotransferase, Ala alanine, DAO D-amino acid oxidase, Glu glutamate, GO glycolate oxidase, GOT glutamate oxaloacetate transaminase, GR glyoxylate reductase, 4-Hyp 4-hydroxy-proline, 4-OH-2-KG 4-hydroxy-2-ketoglutarate, 3-OH-P5C Δ¹-pyrro-line-3-hydroxy-5-carboxylate, LDH lactate dehydrogenase, OAA oxaloacetate, PDH pyruvate dehydrogenase, Pyr pyruvate

Fig. 3

Digestion of dietary protein-bound proline or hydroxyproline (Hyp) in the gastrointestinal tract of the small intestine. Proteases and peptidases in the lumen of the small intestine hydrolyze proteins and large peptides, respectively, to eventually form proline- or hydroxyproline-containing dipeptides. These dipeptides are hydrolyzed by specific proline peptidases (prolidases) to yield free proline or hydroxyproline. Approximately 40% of luminal proline is catabolized by the mammalian small intestine, and the responsible cell types include enterocytes (Wu 1997) and bacteria (Dai et al. 2010). SI small intestine

Fig. 4

Effect of supplementing proline to a chemically purified diet on growth performance of young chickens. Data are taken from Graber et al. (1970). Chicks (the cross of New Hampshire males to Columbian females) were fed a purified diet supplemented with 0, 0.2, 0.4, and 0.8% L-proline for 6 days. The basal diet contained the following amino acids (% of diet): L-arginine–HCl, 1.21; L-histidine–HCl·H₂O, 0.41; L-lysine–HCl, 1.19; L-tyrosine, 0.45; L-tryptophan, 0.15; L-phenylalanine, 0.50; DL-methionine, 0.35; L-cystine, 0.35; L-threonine, 0.65; L-leucine, 1.20; L-isoleucine, 0.60; L-valine, 0.82; glycine, 1.20; and l-glutamate, 10.00. Supplementing L-proline to the basal diet resulted in a linear increase (P < 0.01) in weight gain

Fig. 5

Effect of supplementing proline to a chemically purified diet on growth performance of young pigs. Data are taken from Kirchgessner et al. (1995). Young pigs were fed a purified diet supplemented with 0, 0.35, 0.7, 1.4, and 2.1% L-proline. The basal diet contained the following amino acids (% of diet): L-lysine–HCl, 1.82; DL-methionine, 0.48; L-cystine, 0.44; L-threonine, 1.06; L-tryptophan, 0.26; L-isoleucine, 0.86; L-leucine, 1.58; L-phenylalanine, 0.86; L-tyrosine, 0.86%; L-histidine, 0.56; L-valine, 1.08; L-alanine, 1.67; L-arginine, 0.48; L-aspartate, 3.61; L-glutamate, 2.02; glycine, 1.54; and L-serine, 2.24. Supplementing L-proline to the basal diet resulted in a linear increase (P < 0.01) in weight gain