The ovine conceptus utilizes extracellular serine, glucose, and fructose to generate formate via the one carbon metabolism pathway

Highly proliferative cells rely on one carbon (1C) metabolism for production of formate required for synthesis of purines and thymidine for nucleic acid synthesis. This study was to determine if extracellular serine and/or glucose and fructose contribute the production of formate in ovine conceptuses. Suffolk ewes (n = 8) were synchronized to estrus, bred to fertile rams, and conceptuses were collected on Day 17 of gestation. Conceptuses were either snap frozen in liquid nitrogen (n = 3) or placed in culture in medium (n = 5) containing either: 1) 4 mM D-glucose + 2 mM [U-¹³C]serine; 2) 6 mM glycine + 4 mM D-glucose + 2 mM [U-¹³C]serine; 3) 4 mM D-fructose + 2 mM [U-¹³C]serine; 4) 6 mM glycine + 4 mM D-fructose + 2 mM [U-¹³C]serine; 5) 4 mM D-glucose + 4 mM D-fructose + 2 mM [U-¹³C]serine; or 6) 6 mM glycine + 4 mM D-glucose + 4 mM D-fructose + 2 mM [U-¹³C]serine. After 2 h incubation, conceptuses in their respective culture medium were homogenized and the supernatant analyzed for ¹²C- and ¹³C-formate by gas chromatography and amino acids by high performance liquid chromatography. Ovine conceptuses produced both ¹³C- and ¹²C-formate, indicating that the [U-¹³C]serine, glucose, and fructose were utilized to generate formate, respectively. Greater amounts of ¹²C-formate than ¹³C-formate were produced, indicating that the ovine conceptus utilized more glucose and fructose than serine to produce formate. This study is the first to demonstrate that both 1C metabolism and serinogenesis are active metabolic pathways in ovine conceptuses during the peri-implantation period of pregnancy, and that hexose sugars are the preferred substrate for generating formate required for nucleotide synthesis for proliferating trophectoderm cells.