Interconversion of glycine and serine in a synaptosome fraction isolated from the spinal cord, medulla oblongata, telencephalon, and cerebellum of the rat

Abstract

SUMMARYThe metabolism of glycine was studied in vitro using preparations of synaptosomes (P2) isolated from the spinal cord, medulla oblongata, telencephalon, and cerebellum of the rat. The content of aspartate, glutamate, glutamine, serine, threonine, glycine, and alanine was determined in the crude synaptosomal fraction (P2) from the four regions of the CNS. I n general, the content of the amino acids in the subcellular fraction from the medulla oblongata and spinal cord resembled one another whereas the same was true for the telencephalon and cerebellum. Significantly greater amounts of glutamate, aspartate, glutamine, and serine were found in the synaptosomal fraction of the telencephalon and cerebellum than in similar P2 fractions from the medulla oblongata and spinal cord. On the other hand, glycine had a higher content in the latter two preparations than in the P2 fractions from the telencephalon and cerebellum. [U-lCl-serine and [U-14C]-glycine were readily taken up by the crude synaptosomal fractions. The rate of conversion of serine to glycine appeared to be 18.5, 3.9, 2.9, and 2.7 times greater than the conversion.of glycine to serine in the incubated synaptosomal fractions from the cerebellum, spinal cord, medulla oblongata, and telencephalon, respectively. The conversion of serine carbons into glycine appeared to be 2.5, 1.9 and 1.6 times greater in the synaptosomal fraction from the cerebellum, spinal cord, and medulla oblongata than in similar preparations from the telencephalon. The data are discussed in terms of the possible synthesis pathway for the pool of glycine used as a transmitter.

Glycine: Inhibition from the Sacrum to the Medulla

“…It is interesting that except for glutamate, the levels of glycine in the synaptosomal preparations from spinal cord and medulla are the highest of all the amino acid measured (Osborne et al, 1973;McBride et al, 1973).…”

“…It should also be noted that Roberts and Mitchell (1972) did not find evidence of greater metabolism of the labelled amino acids when the co~ds were preincubtted for 40 minut~s in the presence of either (1 C)-glycine, 1-(1 C)serine or L-(14C)glutamate. For instance, in view of their positive results after the stimulation of the rostral cord, one would have expected (14C)glycine to be released in their experim~nts in which incubations were carried out in the presence of L-(14C)serine, since the latter is readily converted to glycine (Shank and Aprison, 1970a;McBride et al, 1973). Mulder and Snyder (1974) have shown that a glycine-pool labelled by the high affinity uptake system, is more efficiently released from rat spinal cord slices, than cortex slices upon stimulation in vitro by ~.…”

“…Approximately 85% o~ the radioactivity taken up by the incubated P2 ~raction, was ~ound in the sub~raction o~ P 2 containing synaptosomes. In order to circumvent the objection that an exchange o~ labelled and unlabelled glycine molecules occurred, Aprison and McBride (1973) then used unlabelled glycine in their uptake experiments. A net accumulation o~ unlabelled glycine was observed when the crude synaptosomal ~raction (P2) was incubated at 37°C in the presence o~ increasing amounts o~ glycine (0.0375, 0.015 and 0.150 mM).…”

“…The uptake o~ (U_ 14 C)glycine into crude synaptosomes (P2) prepared ~rom the telencephalon and the spinal cord o~ the rat were studied by Aprison and McBride (1973). During the ~irst 3 minutes the rates o~ (u_ 14 C)glycine (8 x 10-7M) uptake by crude synaptosomal preparations (P 2 ) ~rom the spinal cord or telencephalon were estimated to be 0.100 and 0.0243 nmole/mg protein/min, respectively.…”

Glycine Metabolism and Glycine Synthase Activity during the Postnatal Development of Rat Brain

“…It has been generally accepted that the main reaction in the degradation of glycine is catalyzed by the glycine synthase (EC 2.1.2.10) [4] which splits it into C 0 2, N H 3 and methylene tetrahydrofolate (methylene-FH4). This reaction seems to be quantitatively coupled in the central nervous system (CNS) to the formation of serine [5], by the serine hydroxymethyltransferase catalyzed reaction, which can act in some circumstances as a degradative enzyme for glycine [6][7][8], both being enzymes located in the mitochondria [9][10][11][12].…”