Synthesis of glycine from 4-hydroxyproline in tissues of neonatal pigs

Abstract

Glycine from sow’s milk only meets 20% of the requirement of suckling piglets. However, how glycine is synthesized endogenously in neonates is not known. This study determined glycine synthesis from 4-hydroxyproline (an abundant amino acid in milk and neonatal blood) in tissues of sow-reared piglets with normal birth weights. Piglets were euthanized at 0, 7, 14 and 21 days of age, and their tissues were used to determine glycine synthesis from 0 to 5 mM 4-hydroxyproline, activities and mRNA expression of key glycine-synthetic enzymes, and their cell-specific localization. Activities of 4-hydroxyproline oxidase (OH-POX), proline oxidase (POX), serine hydroxymethyltransferase (SHMT), threonine dehydrogenase (TDH), alanine:glyoxylate transaminase (AGT), and 4-hydroxy-2-oxoglutarate aldolase (HOA) occurred in the kidneys and liver from all age groups of piglets, and in the pancreas of 7- to 21-day-old piglets. Activities of OH-POX and HOA were absent from the small intestine of newborn pigs but present in the small intestine of 7- to 21-day-old piglets and in the skeletal muscle of 14- to 21-day-old piglets. Between days 0 and 21 of age, the enzymatic activities of OH-POX, AGT, and HOA decreased in the liver and kidneys but increased in the pancreas and small intestine with age. The mRNA levels of these three enzymes changed in a manner similar to their enzymatic activities. In contrast to OH-POX, AGT, and HOA, the enzymatic activities of POX, SHMT, and TDH were present in the kidneys, liver, and intestine of all age groups of piglets. Glycine was synthesized from 0.1 to 5 mM 4-hydroxyproline in the liver and kidney from 0- to 21-day-old piglets, as well as the pancreas, small intestine, and skeletal muscle from 14- to 21-day-old piglets in a concentration-dependent manner. Collectively, our findings indicate that 4-hydroxyproline is used for the synthesis of glycine in tissues of piglets to compensate for the deficiency of glycine in milk.

Synthesis of glycine from 4-hydroxyproline in tissues of neonatal pigs with intrauterine growth restriction

“…In this regard, it is noteworthy that we recently discovered that NBW piglets are capable of synthesizing glycine from 4-hydroxyproline in a tissue-and age-specific manner. 14 This study shows that the same metabolic pathway occurs, but is less active, in the liver, kidney, pancreas, skeletal muscle, and small intestine of IUGR piglets (Tables 1-4). Glycine is the most abundant amino acid in the plasma of sow-reared piglets with NBW (~1-1.2 mM).…”

“…Another novel and salient finding of this study is that, consistent with enzymatic activities, the rates of glycine production from 4-hydroxyproline in the kidney, liver, pancreas, skeletal muscle, and small intestine of IUGR piglets (Table 4) were generally much lower than those for age-matched NBW piglets in our recent work. 14 For example, the rates of conversion of 5 mM 4-hydroxyproline into glycine in the kidneys of 0-, 7-, 14-, and 21-day-old IUGR piglets were only 15%, 15%, 45%, and 44% of those for NBW piglets, respectively. In addition, the rates of conversion of 5 mM 4-hydroxyproline into glycine in the liver of 0-, 7-, 14-, and 21-day-old IUGR piglets were only 8%, 12%, 25%, and 34% of those for NBW piglets, respectively.…”

“…Of particular importance is the finding that OH-POX activity in the skeletal muscle of IUGR piglets was not detected in the first 2 weeks of postnatal life (Table 1), as compared with the onset of OH-POX activity in the skeletal muscle of 14-day-old NBW pigs. 14 At 21 days of age, OH-POX enzymatic activity in the skeletal muscle of IUGR piglets was only 33% of that for NBW piglets (Supplemental Figure 2). This may be considered as an inborn error of glycine synthesis in IUGR pigs.…”

“…14 In addition, we have constructed a pathway diagram showing the sequence of the required reactions and enzymes. 14 However, little is currently known about this metabolic pathway in IUGR neonates. The current study was conducted with IUGR piglets (an animal model widely used in studies of human nutrition [15][16][17] ) to address this important issue.…”

“…[25][26][27] The experimental details for these procedures have been previously published. 14 Two different amounts of a tissue extract were used for the assay of each enzyme. Maximum activities of all enzymes were measured under optimum conditions.…”