d-Glyceraldehyde 3-Phosphate Dehydrogenases of Higher Plants

Abstract

The d-glyceraldehyde 3-P dehydrogenases of spinach leaf, pea seed, and pea shoot were purified. The NADP and NAD-linked enzymes of either spinach leaves and pea shoots could not be separated. Changes in the ratio of NADP- to NAD-linked activity of the spinach leaf and pea shoot enzymes were observed during both purification and storage of crude extracts. The spinach leaf, pea shoot, and pea seed enzymes differ electrophoretically from each other and from the rabbit muscle enzyme. The pea seed and shoot enzymes contain bound nucleotide cofactor, resist proteolytic attack, have similar Michaelis-Menton kinetic constants and are competitively inhibited by d-sedoheptulose-7-phosphate and d-sedoheptulose 1,7-diphosphate. Charcoal removes the bound nucleotide from the pea seed enzyme but not from the pea shoot enzymes. NADP and NADPH were found to inhibit the reductive but not oxidative reaction catalyzed by the charcoal treated seed enzyme. The function of the pea shoot NADP and NAD-linked enzymes in chloroplast metabolism is discussed in regard to their location and catalytic properties. Although the NADP-linked activity can be assigned a primary, if not exclusive function in photosynthesis, the assignment of a distinct metabolic function to the NAD-linked activity cannot be made at present.