Quantitative comparison of serine enrichment from these two tracers suggests that serine synthesis from glycine occurs via the combined action of the glycine cleavage enzyme system ( GCE ) and serine hydroxymethyltransferase ( SHMT ) . ^^^ These data demonstrate that in primary culture of fetal ovine hepatocytes , approximately 30 % of serine biosynthesis is derived from glycine , primarily via the combined action of GCE and SHMT . . ^^^