Novel phosphoserine phosphatase inhibitors. 
Phosphoserine phosphatase (EC 3.1.1.3) catalyzes the final step in the major pathway of L-serine biosynthesis in brain.  This enzyme may also regulate the levels of glycine and D-serine, the known and putative co-agonists for the glycine site of the N-methyl-D-aspartate receptor in caudal and rostral brain regions, respectively.  Using L-phosphoserine as substrate, the rank order potency for inhibition of phosphoserine phosphatase was p-chloromercuriphenylsulfonic acid (CMPSA) > glycerophosphorylcholine >> hexadecylphosphocholine > or = phosphorylcholine > N-ethylmaleimide > or = L-serine > fluoride > D-2-amino-3-phosphonopropionic acid (D-AP3).  Glycerylphosphorylcholine (IC50 18 microM) was found to be an uncompetitive inhibitor of phosphoserine phosphatase.  Glycerylphosphorylcholine probably binds a novel site on the enzyme since the known allosteric inhibitor L-serine is highly selective for its feedback regulatory site, indicated by the inactivity of 25 L-serine analogs.  Fluoride ion (IC50 770 microM) may bind the active site as has been shown for other Mg2+-dependent enzymes.  The sulfhydryl reagent CMPSA is a potent, noncompetitive inhibitor of the enzyme using L-phosphoserine as substrate (IC50 9 microM) but is > 300-fold less potent using D-phosphoserine as substrate.  Substrate-dependent differences are also observed with the sulfhydryl alkylator N-ethylmaleimide, which inhibits L-phosphoserine, but stimulates D-phosphoserine hydrolysis.  These sulfhydryl reagents may dissociate multimeric forms of the enzyme to form monomers; the multimeric forms and monomers may preferentially cleave L- and D-phosphoserine, respectively.  Phosphorylcholine esters and sulfhydryl reagents may prove useful in determining the contribution of phosphoserine phosphatase to the biosynthesis of glycine and D-serine in neuronal tissue in vitro.