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Low Serine Hydroxymethyltransferase Activity in the Human Placenta Has Important Implications for Fetal Glycine Supply

Centre for the Developmental Origins of Health and Disease (R.M.L., I.T.C., M.A.H.), University of Southampton, Princess Anne Hospital, SO16 5YA Southampton, United Kingdom; Medical Research Council Epidemiology Resource Centre (K.M.G.), University of Southampton, SO16 6YD Southampton, United Kingdom; and Institute of Human Nutrition (A.A.J.), University of Southampton, Southampton General Hospital, SO16 6YD Southampton, United Kingdom

Address all correspondence and requests for reprints to: Dr. Rohan Lewis, Centre for the Developmental Origins of Health and Disease, University of Southampton, Princess Anne Hospital, SO16 5YA Southampton, United Kingdom. E-mail: rml2{at}soton.ac.uk.

Glycine is essential for fetal development, but in both sheep and human pregnancy, little is transported directly from the mother to the fetus, indicating that fetal glycine is derived from other sources. In the sheep, placental conversion of maternal serine by serine hydroxymethyltransferase (SHMT) provides almost all the glycine transported to the fetus. Although mRNA for mitochondrial and cytoplasmic SHMT has been detected in human placenta, it is not known whether substantial placental conversion of serine to glycine occurs in species other than sheep. We determined SHMT activity in human, rat, and sheep placenta by measuring conversion of [3-¹⁴C]serine to ¹⁴C-methylene tetrahydrofolate. Compared with term human placenta, SHMT activity per gram of placenta was 5.1-fold higher in term rat placenta and 24.1-fold higher in term sheep placenta. In sheep placenta, SHMT activity per gram of placenta increased 2.1-fold between mid-gestation and term. In human placenta, placental SHMT activity was similar 8 wk post conception and at term. The low activity of SHMT in the human and rat placenta suggests that, unlike in the sheep, placental conversion of serine to glycine is not a major source of fetal glycine in these species.

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