Fig. 3.

Pathways for providing serine to the brain. Serine can be taken up from the plasma or synthesized from glucose. The synthesis from glucose occurs in astrocytes where part of the 3-phosphoglycerate produced by glycolysis is converted to serine. The serine is transported out of the astrocytes by the Na⁺-dependent ASCT1 transporter. Serine derived from the astrocytes or plasma is taken up by the neurons through the Na⁺-dependent Asc1 transporter. The neurons utilize part of the incorporated serine for PS production catalyzed by either the PSS1 or PSS2 reactions.