Synthesis and release of L-serine by rat astroglia-rich primary cultures

L-serine is known to have important functions in the mammalian CNS other than being a constituent of proteins. It is the metabolic precursor of the neuroactive substances D-serine and glycine, serves as a building block for phospholipid biosynthesis and is likely to be a neurotrophic factor. Based on the observation that rat astroglia-rich primary cultures release L-serine into their culture medium, the biosynthesis and release of L-serine in these cultures has been investigated. Release of L-serine is due to both biosynthesis from glucose and to proteolysis. Amino groups for L-serine synthesis originate from transamination of amino acids that are either taken up from the extracellular space or produced intracellularly by proteolysis. The enzymes of the "phosphorylated pathway" of serine biosynthesis, i.e., 3-phosphoglycerate dehydrogenase, phosphoserine aminotransferase and phosphoserine phosphatase are present in rat brain as well as in rat astroglia-rich primary cultures and their specific activities have been determined. The presence of these enzymes indicates the operation of the "phosphorylated pathway" of L-serine biosynthesis in brain.