SERINE AND GLYCINE METABOLISM IN HEPATOCYTES FROM MID-GESTATION FETALLAMBS

Using stable isotopes of serine, glycine, and glutamine, the metabolis m of serine and glycine was investigated in primary hepatocytes from s ix mid-gestation fetal lambs (mean gestational age = 81 +/- 6 d, norma l gestation = 145 d). Serine production was 6.84 +/- 1.22 mu mol/24 h/ mg of DNA and exceeded serine utilization (3.76 +/- 1.44 mu mol/24 h/m g of DNA) with a resultant net increase in medium serine of 2.58 +/- 1 .70 mu mol/24 h/mg of DNA. In contrast, glycine production (6.84 +/- 1 .16 mu mol/24 h/mg of DNA) was less than glycine utilization (12.10 +/ - 1.78 mu mol/24 h/mg of DNA) with a net decline in medium glycine of -5.44 +/- 2.03 mu mol/24 h/mg of DNA. Of the serine produced, 50.4 +/- 4.3% was derived from glycine via the action of serine hydroxymethylt ransferase (SHMT) and the glycine cleavage enzyme complex (GCS). Incre asing the medium serine concentration resulted in an increase in serin e utilization and sparing of the utilization of other amino acids. Bio synthesis of glycine from serine accounts for only 18.1 +/- 5.6% of gl ycine production, and this percentage is not affected by changes in me dium serine concentration. Using 2,5-[N-15(2)]glutamine as the tracer, an estimated 18 +/- 7% of serine production was derived from transami nation reactions. The specific activity of both cytosolic and mitochon drial SHMT was constant for the duration of the cultures. We conclude that, in mid-gestation fetal ovine hepatocytes, there is net productio n of serine (with glycine as the primary metabolic source of this seri ne biosynthesis) and net glycine utilization. These data suggest that flux through SHMT and GCS accounts for 50% of serine biosynthesis in m id-gestation fetal ovine hepatocytes. The sparing of the utilization o f other amino acids by serine suggests that serine a conditionally ess ential amino acid for the mid-gestation fetal liver.