Glycine and serine catabolism in non-photosynthetic higher plant cells: their role in C1 metabolism

Glycine and serine are two interconvertible amino acids that play an import ant role in C1 metabolism. Using C-13 NMR and various C-13-labelled substra tes, we studied the catabolism of each of these amino acids in nonphotosynt hetic sycamore cambial cells. On one hand, we observed a rapid glycine cata bolism that involved glycine oxidation by the mitochondrial glycine decarbo xylase (GDC) system. The methylenetetrahydrofolate (CH2-THF) produced durin g this reaction did not equilibrate with the overall CH2-THF pool, but was almost totally recycled by the mitochondrial serine hydroxymethyltransferas e (SHMT) for the synthesis of one serine from a second molecule of glycine. Glycine, in contrast to serine, was a poor source of C1 units for the synt hesis of methionine. On the other hand, catabolism of serine was about thre e times lower than catabolism of glycine. Part of this catabolism presumabl y involved the glycolytic pathway. However, the largest part (about two-thi rds) involved serine-to-glycine conversion by cytosolic SHMT, then glycine oxidation by GDC. The availability of cytosolic THF for the initial SHMT re action is possibly the limiting factor of this catabolic pathway. These dat a support the view that serine catabolism in plants is essentially connecte d to C1 metabolism. The glycine formed during this process is rapidly oxidi zed by the mitochondrial GDC-SHMT enzymatic system, which is therefore requ ired in all plant tissues.