Enzymes Related to Serine Synthesis in Spinach Chloroplasts

The three enzymes D‐3‐phosphogIycerate dehydrogenase (E.C. 1.1.l.s), eqglutamate dependent O‐phospho‐L‐serine aminotransferase (E.C. 2.6.1.52), and O‐phospho‐L‐serine phosphatase (E.C. 3.1.3.3.), which together catalyse the turnover of D‐3‐phos‐phoglycerate (3‐PGA) to serine were assayed in chloroplast and leaf extracts from spinach ( Spinacia oleracea ). Relatively high activities of the dehydrogenase and the phosphatase were found in both chloroplast and leaf extracts, whereas the specific activity of the aminotransferase was 10 times higher in leaf extracts than in chloroplast extracts. The results suggest that spinach chloroplasts are able to produce minor amounts of serine directly from 3‐PGA, but that the main part of the phosphohydroxypyruvate formed is exported, and probably used in a parallel pathway to serine outside the chloroplasts. Some kinetic properties of the enzymes were also determined.