EVIDENCE FOR 3 SERINE HYDROXYMETHYLTRANSFERASES IN GREEN LEAF-CELLS -PURIFICATION AND CHARACTERIZATION OF THE MITOCHONDRIAL AND CHLOROPLASTIC ISOFORMS

Serine hydroxymethyltransferase (SHMT, EC 2.1.2.1) is a key enzyme in C-1 metabolism. In order to get more information about SHMT distributi on in leaf tissues, we investigated the intracellular compartmentation of this enzyme using spinach leaf protoplasts. Our results indicate t hat SHMT activity is present in the three major cell compartments, i.e . mitochondria, chloroplasts and cytosol. The mitochondrial activity r epresented about 50% of the total cellular activity whereas chloroplas tic or cytosolic activities each represented approximately 20-25%. The high specific activity found in mitochondria was presumably correlate d with the presence of the photorespiratory pathway in leaf tissue. Su pporting this idea, the SHMT specific activity in mitochondria isolate d either from etiolated tissues or potato tubers was five to ten times lower. SHMT from chloroplasts and mitochondria were purified to homog eneity. Both purified isoenzymes have nearly identical specific activi ties when measured at their optimum pH and similar affinity constants for serine (K-m = 1.3+/-0.3 mM for the mitochondrial enzyme and 1.1+/- 0.3 mM for the chloroplastic counterpart) and tetrahydrofolate monoglu tamate (K-m for the mitochondrial and chloroplastic enzymes were respe ctively 38+/-5 mu M and 45+/-5 mu M). They differed, however by their pH optimum: 7.5 for the mitochondrial enzyme and 8.3 for the chloropla stic counterpart. These values reflect probably the adaptation of the two isoforms to their particular environment.